I found out this information the hard way, so hopefully it will save someone else the trouble that it caused me.

If you, for any reason, have to reload the contents of your blog from a database backup, the second thing you need to do after you install the WordPress platform, and before you import the backed up content into MySQL, is re-install any plugins you were previously using.

Some plugins make tables in your database. When you try to reconnect the database with the new install of WordPress, it’s going to be looking for the plugins that go with the extra database tables. If it can’t find them, your blog isn’t going to show correctly (or at all) in the browser.

You may want to consider keeping a file that lists all your plugins, especially if you have a habit of installing straight from WordPress and not downloading the plugins to your computer first.