Say you're running a version of Joomla other than the latest one, which is currently 1.5.14.  Say you've got a site running, I don't know... 1.5.9.  Clearly it's time to upgrade.  And upgrading is, of course, easy - just download the 1.5.9 to 1.5.14 file from joomla.org, upload it to your server, extract, and you're done.  

Usually the upgrade process is painless and things work fine.  

However, sometimes after an upgrade you may notice that your site looks hideous - that your template's broken and nothing looks the way it did five minutes ago.  If you've run into this, it's usually because you've built your site off one of the templates that ships with Joomla.  The upgrade process will often upgrade the default templates, overwriting your customizations and adjustments and setting the CSS files and .php files back to the way they originally were.  To fix this, you just need to re-upload your customized template files.

What's that?  You don't have a backup copy of your template files? 

If that's the case, you're out of luck and you'll have to rebuild everything in the template.  It shouldn't be the case, though - you should *always* have a full local backup copy of your site, and you should *always* back up your site before upgrading the Joomla core or any major extensions.  

The easiest way to create backups is with JoomlaPack.  If you don't have it, you need to get it.  It makes the process of creating complete Joomla site backups fast and easy.  I install it on every Joomla site I build.  We'll go into JoomlaPack in a future blog entry.  

For now - if a Joomla upgrade breaks your template, it's because you used a modified default template for your site and the files were overwritten.  To fix, just re-upload your modified template files.