I'm confused... do you have a MCD (Mitel 3300) or an SX2000? They are different systems, but older SX2000's are often upgraded using a Mitel 3300 controller and the old SX2000 units essentially become just expansion cabinets.
Are you having an issue with the web interface or the telnet interface? If it's the web interface, system/sx2000 is NOT the default, try system/password.
Sorry, either I do not understand what you are saying and just need some clarification, or you do not understand what you have... but with that said, if you have always used system/sx2000 and that doesn't work and this is a SX2000, we cannot help you, only Mitel technical support can reset this password and only at the request of an authorized reseller. If it is a MCD (Mitel Communications Director/Mitel 3300 or MiVoice Business system) then the password can be reset on some software versions via the RS232 port on the front of the 3300 controller but the exact process can vary based on your software version and older versions require Mitel support as well.