Hi
Many third party tool exist thatcan be used to recover what key was entered. (easyus, magicalbean, etc..) Use the one you prefer.
The computer keep the key you used, but the copy might become un-activated. I would make sure you used a MAK key, a KMS key got time to live but need to be reactived by a central server from time to time.
If the key got problem to activate, try with the slmgr tool from the command line. Some windows build used to have activation problem with enterprise's key.
Use slmgr /ipk XXXX-XXXX-XXXX-XXXXX-XXXXX with the corrcet key and do slmgr /ato
If the key can't be found with the tool I stated, then make sure you have no virus or other thing that can mess up your computer, as after activating the computer don't loose it's key for nothing. At that point you would probably need to contact your seller to have them re-issue you a key.
Thanks