iTunes does not add a passcode during an update so there is no 'default' passcode.
If there is a passcode present now you will have to take the normal actions for a forgotten passcode.
You will need to restore your iPad from the same computer that you most recently synced to.
Read this: http://support.apple.com/kb/ht1212
If you can't do the restore then you will need to try recovery mode.
Read this: http://support.apple.com/kb/ht4097