I will tell story about my problem.
I modify theme.html.. just changing the background of it..
after that... i enable module template .html to update... and then... i dnt like it.. so i change it to original theme.html
looking okey.. if u open the page... if iam trying to login with my login and pass.. after refresh... it just blank page... i turned off the module template .html but nothing happend.. the same thing.. it just blank page... i discover i tried to change my IP address.. or just using a http proxy on the browser... its worked... i dnt know.. what is this ? my IP got banned at my own portal ? can any1 help me or explain me what is happening pls...

Try turning on your debug mode in System Admin --> General Settings and report back what your errors are. PHP debug will perhaps be the best option.

In addition, try uploading a fresh unadjusted version of your theme.html file.