So I've spent many hours in the last few days playing this game and I was just playing when my computer froze... I then turned it off and on and my save file from the last few days is GONE. I'm like freaking out.. is there anything that saves onto my computer or online or something?
I had this problem a while ago. It's a bit convoluted to fix, but you'll get your world back.
You need MCEdit: http://www.minecraftforum.net/viewtopic.php?f=25&t=15522
Go where Sithrix said. Open the folder of a functioning world (if it's a snow map you're trying to save, try to use another snow world), and copy three files: world.dat, world.dat_old, and session.lock. Paste them into the folder of the broken world. Load the world in MCEdit. Use the Player and Player Spawn tools to move yourself and your spawn back to your base, because otherwise they'll be either buried underground or way high in the air. Load the world in Minecraft. Resume playing.
When you reach the previous boundaries of the world, you're going to see some giant flat cliffs. This can't be fixed, it's a result of the game generating land based on the random seed of the world that you copied the three files from. The old world's random seed was lost when Minecraft crashed, because for some reason those three files were zeroed out. Best you can do is bring ladders so the cliffs aren't an obstacle.
Roy will always be Our Boy! SoulSilver > HeartGold. Next case.