Well a few years ago, if my memory is correct, I accidentally submerged my game boy color in water. I think I just left it to dry after that. Anyway, after that it didn't work 100% properly and I'm pretty sure what is happening now is caused by the water damage.

What it actually does is, when I turn it on with a game in it it gets to the loading screen (the word "Nintendo" at the bottom of the screen sometimes glitches here and shows as glitched text or just a black bar) and then it rarely passes this screen. The only time it does pass this screen is if I have Tetris in, and then it's a 50-50 chance as to whether it plays the game or it just fades out from the loading screen and stays black. For all my other Game Boy original games (e.g. double dragon, pokemon red) it doesn't pass the loading screen, and with my gameboy color games (i only have 2) it doesn't even get to the loading screen and just turns on to a blank screen and stays there.

Does anybody have any idea how I can fix this? I've tried a bit of googling and Nintendos website but I couldn't find anything.
Thanks.

EDIT: Also, the sound doesn't come out of the speakers but it works on headphones which I would also like to fix somehow.

I've had some water damage with my GBC too, actually, but mine's mostly fixed (though the screen ghosts a lot more than it used to). Was it just water, or was it something water based (saltwater, juice, carbonated drinks etc)? Because anything impure would leave sediments that would probably cause shorts, which is what I think has happened (from your speakers not working in particular)..

It was a small river so there could have been sediments left in inside it.

have you taken the case apart and, you know, had a look? get pictures if you can. Also yeah, a river could do some nasty things to a gameboy, but it's better than salt (yeah yeah ionic stuff doesn't conduct when solid but good luck getting anhydrous salt crystallising on circuitry ).

It's weird, I can't get inside it to look. All the screws seem to require a screwdriver with three points (I don't know much about tools so don't know if i'm describing that right) and as far as I can see, all of the screwdrivers I have are just a line or cross, not three pointed.

Yeah, you'd need a special screwdriver. Try checking out a chiptunes site: they might sell equipment for modifying this kind of stuff.

Thank you, I'll look into that if it starts acting up again.

For now I seem to have fixed it by using cotton buds to clean the slot on the game boy and the actual cartridges.

You might want o open it anyway, rust could be building up over time.

Hahah, I just used an old screwdriver I didn't need anymore and bent the tip we've got enough of them lying around anyway. But yeah, you probably want to have a look at its insides anyway to see if there's rusting or sediment bridging things, which would explain your speaker issue..
Glad to hear it was the contacts and not anything internal causing problems with your actual games though. (in retrospect it makes sense though because of the difference performance with different carts..)

One time my GBA shoulder buttons were ♥♥♥♥ up so I decided it would be a good idea to rinse it. It broke...

But in a couple days when it tried out, it worked even better.