Can't login after migration assistant. No access to other account
I hope you have resolved this by now, but if not, you can first try booting into Recovery Mode (Command-R for local version or Command-Option-R for internet version).

Once you are in, select Utilities > Utilities which will open a very plain window with a command prompt.

Here, type in "resetpassword" and press enter.

You should now be presented with a more Mac-like window to help you set/reset the password for your available accounts.

If this doesn't work, just come back and we'll try other options.