Access refused on new 27" iMac?

I'm new to macs and bought my iMac a week ago, it's been a learning curve but enjoyable!

I've set up besides mine two other user profiles for my children however it's only my son that uses it at the moment, this morning he couldn't get logged in and we appear to be stuck in an endless loop!

Sequence of events), I used the iMac throughout the day yesterday without any problems and last night I left the iMac to go to sleep / power save but logged into my account (password protected)!

This morning my son attempted to log in by pressing the Guest user button at the bottom (not password protected)!

He would have then logged out of guest using the apple button at the top of the screen and then logged in using his own user profile!

However the mac is sat trying to log into the guest profile and just shows the spinning counter then times out and the screen fades to black!

(I have to suspect that my son may have logged in incorrectly and may have entered an incorrect password or maybe the wrong profile)!

How do I get out of this loop??

Rgds Lee!