It seems I lost a power from one of phases feeding the panel. The pen tester proofs that. It's interesting if I reset the main switch, the power comes back. If I put any appliance (be it a small light bulb) the power disappears on the same phase until main switch is turned -> off -> on. 

I was wondering someone has any theories what could be the issue?

[![enter image description here][1]][1]


  [1]: https://i.stack.imgur.com/HXlcw.jpg