6
Have you checked your group permissions? Maybe no groups are allowed to have module access rights to any of the modules that would display in the main menu?
If you do need to do a clean install, Just backup the database (export the tables in phpmyadmin, copy the queries it gives you, then save it as a text file), then re-upload the XOOPS files, WITHOUT dleting any that are there, just overwrite the ones already there, reinstall xoops, then go back to phpmyadmin, drop all the tables in the database, then open the file you saved with the database query, copy all of the text from said file, then past it to your SQL window in phpmyadmin, click "go" and bam, new install, old data.