Lost saved files/ Need help!

I'm new here and looking for help. I recently got my games out and when I turned them on all but one showed new game. Please excuse my lack of knowledge about this stuff, the games and hacks have come a long way and I lost track of it all. I've been playing since the start so my cartridges are ancient but I had an original mew that I traded for in 96 off a buddy who never did explain really how he got it he just said it was a promo. I swapped mew and my other favorites from blue and red to my silver then to my crystal version and now the only games showing a save file are sapphire and newer. I wouldn't care so much if it wasn't for losing mew. I was never able to trade it to sapphire. Can somebody explain to me how this happened?? I understand internal batteries running dry but how did my save files get lost? How big of a deal is it to get another mew??

It's a common problem with Gold, Silver, and Crystal games. Due to the constantly running clock on the games, the internal save battery depletes even if the game isn't being used and that causes the file to get erased and no matter how many times you create a new file, your file will just get erased after turning the game off. Replacing the battery is hard. There are a few tutorials on Youtube that shows you how to replace a battery in your Pokemon cartridge.