Ranayna: Assuming you are on Windows 7 or 8, the path on XP is slightly different.
First make sure Terraria is not running.
Go to the following Folder: C:\Users\<Your Username>\Documents\My Games\Terraria\Players
If you have not already done so globally, turn off "Hide extensions for known filetypes" in the Folder Options.
You should see a folder and two files for each of your Characters, a <Charname>.plr, and a <Charname>.plr.bak (two extensions, this is your backup file!)
Rename <Charname>.plr to <Charname>.plr.old or something like that, or move it to another folder, or (not really recommended) just delete it. Afterwards, remove the .bak extension from the other file.
Start Terraria and try to load your Character. If it still does not work, rename the <Charname> folder to something different, and try again. This will reset your minimap though.
You might have lost some stuff, but this should get your Character back. If your Worls have the same Problem, the solution is similar, just locate the corresponding files in the Worlds folder. Those will have <Worldname>.wld and <Worldname>.wld.bak as extensions.
I just realized one of my single player worlds won't load now..........