Turn on the error-reporting in Joomla, it should give you some clues...
Usually it's caused by one of these:
1. You ran out of memory for PHP to run.
2. Low diskspace.
3. Bad SEF or .htaccess.
4. Missing files.
5. Bad ownership and permission issues.
6. Compatibility issues between plugins.
7. Bad webhost.
8. Your sites have been hacked.
I can't help more than this.
Please reply with a direct link to the issue & create a new thread for each new issue.
A template is only as good as the content that goes into it - DanG