So many possibilities here!
Check your /system/config.php file. There is all the data about connection to your database as this does seem to be a database issue. Confirm that the config file matches to your database name, user, type etc .
Do you have other errors in the error log?
Have you checked that your permissions are set correctly?
Have you checked to see if your system requirements are set correctly they can be found here:
https://abantecart.atlassian.net/wiki/spaces/AD/pages/3768390/AbanteCart+System+RequirementsYou can try turning off (not deleting, just off) your extensions and seeing if you still have the problem. Clear your system cache afterwards.
If you don't have the issue, trying turning one at a time on until you get a recurrence.
If you determine is an extension contact the developer of that extension and give them the details of your issue. Including what additional extensions you have installed and ON when you encountered the issue.
Extensions in the marketplace usually meet the standards of working with default code - but there are quite a few different devs in the MarketPlace and there is no way for them to know what any other extension may be altering default code.