Lost my Activation Key After resetting windows 10.

The reason it is not activated as you have stated is because you now have Windows 10 Pro, when the edition the device has a licence for is Windows 10 Home.

How did you Reset. Did you use the Reset function within Windows 10. It should have reset to Home edition. If you are sure you have Pro now, you will need to clean install to get back to Home. This will wipe all data on the laptop. Is there a recovery routine provided by the manufacturer? but that will still remove all data from the laptop.

You need to backup all your personal files to an external source.

You can use the Media Creation Tool to create boot media using the link you posted. Use the Custom option during installation and delete all windows partitions.

If you need any more detail then post back.

You are going to need to do the following in order to downgrade to Windows 10 Home Single Language:

1. Open Registry Editor(WIN + R, type regedit, hit Enter)

2. Browse to key HKEY_Local Machine > Software > Microsoft > Windows NT > CurrentVersion

3. Change EditionID to Home (double click EditionID, change value, click OK). In your case it should at the moment show Pro

4. Change ProductName to Windows 10 Home. In your case it should at the moment show Windows 10 Professional

5. Browse to key HKEY_Local Machine > Software > Wow6432Node > Microsoft > Windows NT > CurrentVersion, change the same two values as in steps 3 and 4 above

6. Close the Registry Editor

7. Insert the Windows 10 Homeinstallation media, do an in-place upgrade by launching the setup from desktop (not booting with Windows 10 Home install media)

8. When asked, enter your validWindows 10 Home product key and select what to keep(settings, personal files and apps, only personal files or nothing)

9. Windows 10 Home in-place upgrade will be done now.

Note: How to verify if you downloaded or installed the Windows 10 Anniversary Update

Select the option Upgrade this PC now if you want to upgrade right away, of if you want to download a ISO image or create a bootable copy on a USB thumb drive, choose Create installation media for another PC then click Next

The reason it is not activated as you have stated is because you now have Windows 10 Pro, when the edition the device has a licence for is Windows 10 Home.

How did you Reset. Did you use the Reset function within Windows 10. It should have reset to Home edition. If you are sure you have Pro now, you will need to clean install to get back to Home. This will wipe all data on the laptop. Is there a recovery routine provided by the manufacturer? but that will still remove all data from the laptop.

You need to backup all your personal files to an external source.

You can use the Media Creation Tool to create boot media using the link you posted. Use the Custom option during installation and delete all windows partitions.

If you need any more detail then post back.

Okay. ! No i havent used the inbuilt reset function, i have used an external bootable device to boot to Windows 10 pro. But what if i first backup all my data to an external drive and then upgrade it to Windows 10 home single language will my device be digitally licensed as it was before when i had bought.?  No i dont know about whether HP had provided me with any recovery routine for this kind of problem.? If you have any idea about this (Recovery routine by manufacturer), let me know about this asap. 

You are going to need to do the following in order to downgrade to Windows 10 Home Single Language:

1. Open Registry Editor(WIN + R, type regedit, hit Enter)

2. Browse to key HKEY_Local Machine > Software > Microsoft > Windows NT > CurrentVersion

3. Change EditionID to Home (double click EditionID, change value, click OK). In your case it should at the moment show Pro

4. Change ProductName to Windows 10 Home. In your case it should at the moment show Windows 10 Professional

5. Browse to key HKEY_Local Machine > Software > Wow6432Node > Microsoft > Windows NT > CurrentVersion, change the same two values as in steps 3 and 4 above

6. Close the Registry Editor

7. Insert the Windows 10 Homeinstallation media, do an in-place upgrade by launching the setup from desktop (not booting with Windows 10 Home install media)

8. When asked, enter your validWindows 10 Home product key and select what to keep(settings, personal files and apps, only personal files or nothing)

9. Windows 10 Home in-place upgrade will be done now.

Download the Media Creation Tool Now

Right click the MediaCreationTool.exe file then click Run as administrator.

Note: How to verify if you downloaded or installed the Windows 10 Anniversary Update

Select the option Upgrade this PC now if you want to upgrade right away, of if you want to download a ISO image or create a bootable copy on a USB thumb drive, choose Create installation media for another PC then click Next

I havent purchased windows 10 home single language, my device was digitally licensed with the mentioned OS when i had bought it. So what do i do if it asked for a product key while installtion. ?  Should i skip that step,  so that after installation my device will automatically recognise the product and will activate it (Without any activation key). And using windows media creation tool should i directly boot my device or do i create a bootable USB flash drive to boot it to windows 10 home single language.

I have no experience with HP recovery routines.

You only have a licence for the original Home edition. If you have installed Windows 10 Pro then you will need to clean install using the routine from Andre. You need to create USB boot media then using custom install delete the existing windows partitions. (there is no "upgrade" route from Pro to Home) You will need to backup all your personal data.