You'll probably want to open the machine up and check to see that the RAM sticks are properly seated in the slots. Unfortunately RAM sticks can go bad and die.
Fortunately if it is bad RAM, the sticks are cheap.
Here's a set for $15.
http://www.amazon.com/Sampler-MPC2000-M ... B004O3XKD8Also while you have the unit open look for any ribbon cables and other connectors that may not be seated properly in their slots/connectors.
Make sure to touch something metal you have around with all 10 fingertips to discharge any built up static electricity you hands my have. On the same note don't have just socks on a carpet if for example you do this on a coffee table.
This MPCStuff.com video will show you how to open the machine and what to expect looking at the interior. Mike @MPCStuff always used an ice cube tray to keep the many screws you have to remove separated. It is way too easy to end up with an extra one or two on re-assembly and just be scratching your head.
http://www.youtube.com/watch?feature=pl ... jvc#at=105Labeling the tray or whatever you use for the screws is good too because if you mix up the smaller ones the case won't go back on properly.
There is a Memory test mode on an XL but if you can't get to the Main screen on boot up I'm not sure you'll be able to access it.
http://www.mpc2000xl.com/secret.htm