An old EL... Power it up, connect with a straight through serial cable, set the speed to 9600 8N1 and open a VT100 terminal emulator (TeraTerm works well) and press enter several times...
Select VT100
Select CDE or Maintenance
Username: installer
Password: (the factory default password is well documented, not sure I can post it here though)
If that doesn't work, and you don't know the password, you may be in a situation where you need to factory default the system... If you do you are in trouble. The 200 is similar to other Mitels and it has an options list and a password tied to the sysid of the system, if you factory default it you need that AMC record information to key it back in manually.