It sounds like you edited the default twentyten theme reather than creating a child theme for it or at least renaming it. The old version is now gone and the only way to get it back is by restoring from a backup.
If you didn’t make a backup ask your web hosting company if they have one.
I made a backup, but I cant open it. I’m using stuffit on a mac, but when I try to unzip the .sql.gz file it wont let me open it. I know if I just import the backup it’ll go back to normal. Any suggestions?
As its a sql backup, that is a backup of the database. Importing won’t put any overwritten files back. If you restore it, it will not help as it will only get your database back to the earlier state. Do you have a file backup?
I don’t think I have a file backup.
Ask your hosting company if they take file backups and are able to get a previous version of your wp-content/themes folder back.
