I have a problem since when calling www.mydomain.com/zp-core/admin.php zen displays the index.php and I can't find any other way to log in. Same thing happens when I switch to default Zenpages theme. How can I call login panel?

That is the right URL. What Zenphoto version? Any server errors?

The strange thing is that I used to log in Zenphoto and everything was working perfectly. Suddenly there are problems.

Zenphoto Setup v1.2.6[4335]
It's under www.top-parkiet.com

There are no php errors displayed on the page.

Sure your browser has cookies enabled? Besides, Zenphoto 1.2.6 is quite old...

I took a quick look and this is some issue on your theme setup. You use some strange frame construct there and the Zenphoto install is apparently not under that domain you posted but a subdomain. The link "Galeria" shows this /kiwus.vizz.pl/top-parkietindex.php?album=zdj-cia There is apparently a slash missing before index.php.
On kiwus.vizz.pl/top-parkiet/zp-core/admin.php you get your login form as expected.

Why that is so you have to figure out yourself as this is not a standard theme or setup at all.