Hello;
try to login to your ACP typing
http://path/to/your/gallery/admin/. If you can access to your administration, check latest submissions.
Go to "Edit images", on the left. Then, at the bottom of the right frame choose
Order by: Date and
Descending.
Then you will find a PHP file recently uploaded. You can there delete the file and the user who uploaded it. Returning to your gallery homepage would then be possible.
You may also check in what category the file has been uploaded, and check by FTP or your Cpanel file browser if it remains some "alien" files in
your_site/data/media/cat_number/.