After reinstalling Zotero, you may see a yellow exclamation sign appear over the Zotero icon in the status bar and see one of the following messages when clicking on it:
Zotero user database version is newer than the SQL file.
This version of Zotero is older than the version last used with your database. Please upgrade to the latest version from zotero.org.
These messages result from trying to use a Zotero database from a later version of Zotero in an earlier version of the software. For example, if you previously installed Zotero 2.0 and later installed Zotero 1.0 into the same Firefox profile, you would get this error. This applies to minor versions as well, so installing Zotero 2.0b7.4 over Zotero 2.0b7.6, or a release version over a more recent development build, would produce the same error.
The best solution is generally to reinstall the same version you were using previously—in most cases, this will simply require going to zotero.org and installing the latest available version. (If you were using a development build, you will need to install the latest dev XPI.)
If you have a backup of your Zotero data directory from the previous version and would like to revert to that version, you can also, with Firefox closed, move your current Zotero data directory and replace it with the backup.
If you don't have data you care about, you can keep your current Zotero version by closing Firefox and deleting the Zotero data directory from your Firefox profile.