While I was saving on my newest character today, I accidently saved over my level 37 main character who has done every daedric quest, the thieves guild questline, most of the main quest, and cleared many dungeons who i've played over 80 hours on. I didn't panic though because I heard there was a way to back up the save file so it will still be there but I'm not sure if it's possible or how to do it.

Does anyone know if I can make my main character's save reappear or will I have to start all over again?

Go back to a previous auto save maybe? I did something similar a while back too. I think it saves 4 auto saves so hopefully 1 of them will be your main one

I haven't played my main for a week so all my auto saves are for my newer character

Go into your My Documents\My Games\Skyrim\Saves folder. Sort by Last Modified date to see the newest. The old "overwritten" save is there with a .bak extension instead of .ess.
Just rename it something else with an .ess extension.

I have about 3 autosaves that are .bak. When I rename it to .ess it says the file may become unstable.

Ignore this. Also change the filename before the extension it so it doesn't end up the same as another file...

Windows says that? Just ignore it then.

It doesn't work, I still have 2 save files named with my newer character's name. I renamed some of the .bak ones to .ess but when I try to load them up they are just my newer character.

If you are using Windows 7, right clic the Saves folder and select restore previous version. From that menu, you should find some options to help you.

Next time, back up the save folder before starting a new character!

Errr.. I know it's probably no help to you, but for others reading this thread - this is why you should always keep MULTIPLE saves.
Personally, I always create a new save for my characters at the end of every day's play. That's apart from the quicksave which I do basically every time I've done enough that I don't want to repeat.
When I build up a whole bunch of saves, then I can thin them out a bit by dumping older ones now and then.

I'll never understand why people rely on one single save in games like this, with such a huge time-investment in every character.
I remember when I started computer studies in college (a couple of centuries ago), there was a sign on the wall which said:
"Thou shalt backup and backup, until thou art blue in the face from backuping".
Sound advice, and I live by it.

I'm not using 7, just Vista ;_;

Just a tip. I always turn of Windows hide File extensions. By default that is hidden. It makes renaming alot easier as windows doesn't screw you over when you rename and such. Then you can see exactly what the file is named. Not only that is an executable, text file or whatever.