Most problems are solved by one of these steps. Try them in this order before you ask for help. They're all in ACP > Modules > Core > System Core > Tools.
Integrity Check checks that every module and skin is installed properly, and that the database tables are as they should be. Run it after you install or update anything, and whenever something behaves oddly.
Verify Modules, Verify Skins, Verify Database and Verify Languages are checked by default, and are right for most problems. Click Run Integrity Check. The options below the warning (Optimize Tables, Repair Tables, Repair Modules) are for when you need them, such as a crashed table.
Jamroom caches pages, templates, CSS and JavaScript so your site is fast. If a change doesn't show, such as a template edit or new CSS, reset the caches.
Reset Caches lets you choose what to reset: Database Items, Language Strings, Form Sessions, JavaScript Files, CSS Files, Sprite Icons and the template files. Check them all if you're not sure.
System Check shows your server's setup: PHP version and settings, MySQL, disk space, folders Jamroom must be able to write to, and tools such as FFmpeg. Anything that needs attention is marked in red or yellow. See Run the System Check.
The error message is the most useful thing to include if you ask for help.
If the site doesn't load at all, often because of a crashed database table, use the repair script:
repair.php.html to repair.php.yoursite.com/repair.php in your browser.repair.php.html when you're done. Anyone could run it while it's named repair.php.Until you rename it, the page just says "This file must be renamed to repair.php to function!".
If even the repair script can't run, delete the folders inside data/cache on your server, for example with FTP. Jamroom rebuilds them as needed.