Is your WordPress theme broken? Typically, you can troubleshoot exactly what the problem is in just a few minutes.
The first thing you should know is that WordPress works great out of the box! If you're current theme is broken, things are not displaying properly, pages are not fully loading, pages are not found or 404 error, or clicking or certain things breaks stuff – here are the steps you should take to get your theme working again.
1. Deactivate all plugins. Reload your blog page with the problem. Does it work now? Activate all your plugins one by one, linking your blog page each time – and the one that breaks it is the problem (deactivate and delete it). If this does not work move on to step 2.
2. With all plugins still deactivated – activate and revert to the "default" WordPress theme. Does it work now? If so – you've hacked your theme and broken it. Try downloading a fresh copy (or just switching themes). If this still does not fix the problem try step 3.
3. Links to your own blog pages do not work? Go to "Settings-> Permalinks" and just hit "save" to update them again. Do they work now? Move on to step 4 if you're still having issues.
4. Blank pages, blank WordPress dashboard login – or can not login: check your wp-config file first and make sure it's there and has the right connection information. Reupload all the WordPress core files again to see if one was corrupted, and check your theme page again. Try step 5 if this still has not fixed it.
5. If you're still having problems, the first thing I would do is to contact your web host and ask if there were any recent upgrades to the server you're hosted on. Sometimes upgrades to Apache or security rules can screw up WordPress sites, and you might not be the only customer affected.
If none of the 5 steps worked search the WordPress Support Forum and Codex – there might be someone with the exact same problem as you, that has already found a solution!
98% of the time when your WordPress theme breaks it's something simple, and the steps above will get you back in business again. If you're already in there mucking around with themes and plugins – I would also advise that you read my WordPress Security Guide for DIY info on how to secure and harden your blog. If your WordPress blog has been hacked and you need immediate help, follow that link for information on asking me for assistance.