I have an old Mitsubishi big screen and it comes on then shuts right back off what is my problem

Mitsubishi will have a VS, WS model number.

Most will have a date made on the rear and ones after about 1998 have a self check.

If green power light comes on for a few seconds and goes off:

On the front of the set (not the remote) press and hold BOTH THE MENU AND INPUT (input on some models is Device) FOR AT LEAST 5 SECONDS AND RELEASE BOTH.

Power light will blink a 2 digit error code like: 2 blinks, 2 blinks a pause and repeat it several times.

If you have unplugged the set that clears the error code and you will have to plug it in and try to turn on again--then do the check.

Let me know what error code you find and I will tell you what it means--2,2, is the most common one.