How to get product key of old OS (Win 7) after upgrading to Win 10?

Before 2 Months, I was using Windows 7 Home Premium in my Sony Vaio Laptop which is running absolutely fine. After the release of Windows 10, I upgraded and then clean install my Windows 7 to Windows 10. But after the upgrade, my laptop becomes too slow. It takes about 10 minutes to boot and 5-6 minutes to start the application.
So now i want to go back to windows 7 but now since its over 1 month, I cannot rollback to windows 7. So I want to perform the clean install the same old Windows 7 but I lost the Windows 7 product key that comes with my laptop. Also the sticker at the back of laptop where the product key is printed is erased.
Now in this difficult situation, how can i get the product key of my old windows 7 in order to install it? Is it possible?
Please help me.
Badly regretting for upgrading my Windows 7 to Windows 10.

showkey plus from this forum:
Showkey - Windows 10 Forums

Cliff S said:

Sorry but that won't work if he clean installed after upgrading and has an OEM license. It only say you have an OEM Marker.

It only shows the generic install key for Win10
On windows 7 machines there is no key in the mother board BIOS.

Oops. I missed the OP where he said he did a clean install after the upgrade. Yep, you are correct, the previous Windows 7 Product key is gone - unless the OP made a backup image of the Windows 7 or first Windows 10 after upgrade, which they would just restore, so there probably was no backup done.

SunilKunwar said:

Also the sticker at the back of laptop where the product key is printed is erased.

That is the only one you need for 7. You was ripped-off.

Later versions had a key in BIOS but 7 didn't.

If you have not bought a legit version (and someone has scrubbed off the original OEM number) then you'll have consider buying a new license from MS.

I get given lots of laptops to "repair" which people have bought from a "reputable" seller (from a local shop or ebay or whatever). In fact I've never seen one that was genuinely activated. Never.

As you can't validate your 7 you can't expect your upgrade to be valid.

Kari said:

The thing is, even if you could retrieve the product key of the underlying OS, it would be useless. If a valid 7 or 8.1 was upgraded to 10 and the 10 automatically activated, the underlying 7 or 8.1 key is flagged as used and can't be used to activate 7 or 8.1 on another machine.

The key of an underlying qualifying OS is gone and invalid as soon as the upgrade to 10 has been activated.

100% complete and utter nonsense.