If your site is going wrong first try to remember if you have recently made something yourself. For example, have you installed a module or a hack ? If that's the case, try to desactivate or uninstall the module. If it's a hack, restore the original file. If it solve the problem, report the problem to the module's author (or to the hack's author)

If you are not in this case, go in the general preferences of your site and activate the php debug mode then report the error messages to a support site. Error messages are located at the bottom of the page. If you can't see any error message, activate the mysql's debug mode and report also the problems to a support site.