I've installed zen photo a few months ago on a couple sites, today it's not working. The setup page comes up, but it's not asking for db connection info

More info please.

If your setup page comes up, then most likely you did not remove the setup files after installation. This is recommended. There have been other similar reports. So if yours is the same, someone has hacked your zp-config.php file.

Please remove that file and run setup again filling in the correct information. Then remove the setup files so this does not happen again.