Hi tessmila,
Unfortunately, your only options with something like this are:
1) Use FTP to edit the file and remove the change you made. You could try using Filezilla and they have documentation that might help on how to use it.
2) If the website's host has a file manager when you log into the hosting account, you could use that to edit the file and remove what you changed.
3) If neither of the above is an option, the only other alternative would be to contact the web host directly and ask their support for help.
That .htaccess file is pretty crucial to your website. If something goes wrong with that, it will prevent the site from working and so you won't be able to get back in and edit it through the website (e.g. through WordPress).