What model of Dell? If a laptop have you checked under the battery?

yes that was stupid of me no to include  models information! Optiplex  980 <ADMIN NOTE: Service tag removed per privacy policy>

Thank you for looking

and of course its activation not serial number

The system was shipped in 2010 with Windows 7 Professional... it should have a COA affixed to the top or back of it. If this COA has peeled off theres nothing that can really be done to recover the key.

The key itself doesn't matter, you can clean install Windows 7 Professional using Dell Installation Media with Dell OEM System Locked Preinstallation (Dell BIOS based activation) see here for details:

http://dellwindowsreinstallationguide.com/windows-7-sp1-iso-download/

After that you can perform an inplace upgrade to Windows 10 TH2 Professional (do this within the next 10 days to get the free upgrade) to make your system a Windows 10 Pro Device:

http://dellwindowsreinstallationguide.com/upgrading-to-windows-10/

This will register your systems hardware profile with a Microsoft Product Activation server. Once this has been done if you ever need to reinstall Windows 10 Pro, you can install without a key and your system will automatically reactivate when online after re-establishing contact with the Microsoft Product Activation server:

http://dellwindowsreinstallationguide.com/download-windows-10-oem-and-retail-iso/