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you accidently deleted the files on the site? But the database is still there?
If that is the case, you should just upload the XOOPS package with the modules, you used to have.
After that, just make sure that the information in mainfile.php is correct (paths, database settings) and you should be good to go.
All the installation does is write the information to mainfile.php and create the database tables, but if you have already the database, all that remains is the database, which you can edit manually.