Need Help – Posts showing as blank

I made a one character change to file single.php
After I made the change, whenever I click on a Post on the website, it appears as though it has loaded it, and the webrowser says “Done”, but the page is blank!
I changed the file back immediatley, but it is still having the same behaviour.. I feel terrible because I’m just helping somebody with their website, and now I have rendered it virtually useless 🙁
Any help would be appreciated beyond belief.

Thank You in advance for your help.
Barbara