If your system is password protected and you do not have the password, then you have a problem. There is no back door, however there is a procedure to disable the password. It is not for the novice or faint of heart. You must take the system off line, connect with hyperterm with an NEC programming cable then use the MOC mode emulation to communicate with the system. Then go to the password enable command and turn it off.
I suggest you contact the previous vendor or tech and try to get the password. Try to find an experienced tech. If this is not possible, your options are to default and reprogram, or use the procedure mentioned below. It's best to let an experienced tech do this.
First, verify you have a working version of hyperterm. It's difficult to find these days.
Verify you have an NEC programming cable.
Place the rotating sense wheel on the CPU on the 5 position, then press the reset button.
With the system off line, you can connect with hyperterm at 9600bd.
Use the MOC/hyperterm emulation buttons to get into the program. (see attached)
Go to CMD E9 data enter 9 data enter change the 0 to a 1, execute. (E9>9>1>exe)
When done set the sense wheel back to 0. The system should reboot with the password inactive.
Read through and thoroughly understand what to do before doing it. Mistakes can render the system inoperable.