The SQL connections are stored in the server.xml file. The changes to the file must be put back in when you upgrade as the file is overwritten during the upgrade process. Any changed files should be backed up by the upgrade process, so you should be able to see what you had in there prior to the upgrade.
Davin's answer is the most likely scenario if you have just done an upgrade. Any custom configuration you do in Confluence (including adding datasources to your server.xml file) need to be remembered in your upgrade process documentation and then re-applied after the upgrade.
And if you are still having problems open a support issue with us.
Ok, that's clear. What data sources have you made available in the Confluence configuration? If it was working on a previous version, then my best guess is that you have lost/overwritten the datasource configuration the plugin is using when you upgraded. You'll need to look at the config you had before.
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