If the default password ***** been changed then you will need to reset it by removing the CMOS battery for 24 hours.
I assume you have the system disconnected from the power supply?
If removing the battery does not work then there must be some residual power still stored inside it - you can try leaving it for longer, or contact Interlogix directly for the master reset code for your particular unit:
The brand is now owned by Interlogix - did you try them?
And they are unable to reset it for you?