Hi and Welcome to the Forum.

I would suggest re-uploading the original 'admin' dir. It should replace / overwrite the files you have now and (hopefully) fix the problem. If it doesn't then it could be a problem with your database. In that case you should delete your database and recreate it using the same database name, database username and password. Then upload the "zc_install" dir. and install like you would a new Zen Cart.