Whenever I take the cartridge out of the Gameboy, I have to start the game over. Why?
Its probably messed up. Old games mess up easily and this has happened to me before. The best thing is to just buy another one if you want to play the original again, or you can buy HG/SS which is basically the same thing with some changes.
For Gen 1 and Gen 2 if you save and it does not keep the save, thus making you start over. Your cartridge's battery is probably dead and just needs to be replaced. As for with gen 3, the battery in this one just controls timed events.