My site is blank (after installing theme)

Hi,
I have an older version of WordPress, but I installed a new theme on it by accident. The website then read “fatal error”, and I couldn’t edit it at all. I used the FTP to rename the new theme something else, hoping that the site would go to the default theme, and I could start working on it again. But now when you type in the URL (), nothing comes up at all. Any suggestions on how to fix this?

Thanks!