Funny, I've answered this same exact question before.
Did you search the boards?
Try this out, as mentioned in this thread:
Originally posted by bradgrafelman
Try this:
RewriteEngine on
RewriteBase /
RewriteRule ^([A-Za-z_0-9-]+)((/)|())$ user_home.php?user=$1
[/B]
Just modify the user_home to the appropriate script name.
EDIT: You, er... removed the htaccess and it still redirected? Erm, try doing cat .htaccess in the shell (make sure you're in the correct directory). What happens?
EDIT2: Just tried that RewriteRule (once again) on my own server - worked flawlessly.
EDIT3: The only way I can restore a normal .htaccess file on a friend of mine's website (his webhost did not provide shell access, and the FTP server/client, naturally, hide's files prefixed with a period) is to make a file called .htaccess on my comp that has nothing in it - a blank file. Then, I upload that to the directory that had the 'old' htaccess file. Whether it asks you to overwrite or not, it should replace your htaccess with a blank file, therefore resetting all of Apache's default configurations.