Bing - My Pokemon Crystal keeps deleting my save file!?

If you're playing on the actual cartridge, it means your battery went dead. You have to find someone on eBay who replaces batteries, and send it to them to fix it, or, you can get a Gamebit screwdriver and a battery (the specific type I forget), and do it yourself. You also need to know how to solder if you want to do it yourself.