I cant enter firmware pin number when requested

My place of work have given me an old imac they no longer needed, unfortunately it doesnt seem like they removed all the controls they'd previously had on it :(.

After setting up a user area and restarting it asked me for what i now think is a firmward pin code. Not knowing what it was i guessed what it might be and it obviously said it was wrong and locked my out for a few minutes.

now.. my employees have finally dug out what they think might be the pin code, however i seem to be in and endless loop of not being able to enter it (or its just wrong and I dont know it).

After the time lock finishes it says 'wrong password. Try again' but then doesnt give me any way to re enter it! :( . So i restart the mack and it gives me the firmware lock screen with the padlock where i enter the pin code ive been give - it then continues to the startup progress bars (does this mean its the correct pincode?). But then once getting past those progress bars it takes me back to the 'your computer is disabled. Try again in 60 minutes' screen :( :( .

Does this mean that the code is not correct or is something else wrong? Its odd that after the time runs out it gives me no way of re-entering the code anymore.

Im really hoping this is solveable as my sons now got his hopes up and im worried of being stuck with a bricked imac :-/

thanks for any help you can give!

Nick