well your gallery is closed so I cant see anything to suggest what the issue might be.
Have you checked your settings your paths etc?
Nothing has changed it appears to me your server settings have changed. All directories should be 755 expect files and users as noted in the install instructions. As stated something has changed whether it be you setting permissions incorrectly or server being upgraded your host changing something. Nothing happens for no reason.
If you open the gallery or provide us through the contact us link with an admin login to see your showimage page etc I can make a suggestion of what might be wrong.