iPad stuck with iCloud I am trying to reset my iPad, but now I am stuck at the LogIn screen to the iCloud.


  I can't remember either my password nor my username. What can I do to
  activate the iPad and/or get my username/password combination?
That sounds like you had "Find my iPad" enabled in iCloud. It's made to protect your iPad against theft (or at least make it potentially useless to thieves). You won't get around logging into your iCloud. 
If you also have it on a Mac you could check if your password is stored in the Keychain (located in Applications/Utilities).
Else you'll have to reset the password, you can do that on Apple's iForgot page. If none of the options on that page help you, you'll have to call Apple and they'll try to verify that you're the owner of the account and help you reset your password. 

As a hint: your Username is usually you're E-Mail address.