The display panel on Panasonic CD stereo system SA-PM 21?

It's a big question - how can you fix it?

If you had the confidence and equipment to dismantle it and diagnose the fault you would not ask how you might fix it....

It sounds very much like a fault in the power supply area but that's just a guess based on the display not lighting up.

Your realistic options are:
Take it back for a warranty repair if you can
Find a specialist to look at it - may not be cost effective
Buy another one

Best of luck...
January 2015