Q: how to activate ipad after factory reset if icloud account details has been forgotten?

There is no workaround; you will need these credentials. This is an anti-theft feature called Activation Lock.

You can try using www.iforgot.com to determine the Apple ID, then reset the password.

Then what you would need to do is contact Apple Care.  Let them know you need assistance in removing activation lock, and you have proof of purchase available. 

AppleCare US phone number: (800)-694-7466

Contact Apple for support and service

http://support.apple.com/kb/HE57

Call AppleCare and ask to be transferred to the Account Security Team. They will work with you to recover the account you set up the iPad with.

Are you saying you had an iCloud Apple ID and a separate Apple ID?

hi

i have the same problem with my iPad.

how do you unlock the iPad?

thank you

If you can't remember your id, are you still logged into the account anywhere so that you can look it up ? Or do you have any of the account's downloads in your computer's iTunes library so that you can do 'get-info' on them and see the id, or have you tried : If you forgot your Apple ID - Apple Support ?

If you still can't find it, and if you were the original purchaser of the iPad (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the iPad from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it