I get get this code at the top of page when I try to log in:
'Warning: session_start() [function.session-start]: open(/home/users/web/b1635/xyz/contacts/cgi-bin/tmp/sess_62ff2c96d2b18ad84c66f19279d6a2af, O_RDWR) failed: No such file or directory (2) in /hermes/web11/b1635/xyz/zenphotos/zp-core/admin-globals.php on line 15'

I deleted the contacts folder, it doesn't have anthing to do with my Zenphotos project/folder. Any ideas how to fix it?

The hosting company fixed it for me. If you run into a similar error they said that the session.save_path needed to be set in the php.ini file.
I didn't ask how it became "unset". Probably when I deleted the other php folder "contacts". As you can tell I'm not a programmer.

Thanks for the update. This issue has popped up from time to time. You are the first to identify what causes it.