If it is not UV, then, place some VIM on a paper towel or J-Cloth and test clean a corner to see if it works.
Note that VIM is a very, very, very mild liquid abrasive. I used it to clean an old Amiga 4000 before ebay sale and it has done wonders. Note that after using VIM, you will need to clean it off, I tend to spray some Windex on a Jcloth or paper towel and wipe off a few times.
Warning, always apply the cleaner to your cloth/tower, never spray directly on the hardware, or, it will get into the electronics.
If the test shot of VIM works, you will need to disassemble your front panel for proper cleaning. If it doesn't work, then you plastic has browned due to sun exposure. There are more fancy chemicals and procedures to clean that, however, these methods probably aren't safe for the membrane keypad.
When I say VIM, I mean the squeeze bottle white liquid creamy stuff, not the spray bottle. See photo: