Can't login

Hello, sorry if I write this to the wrong forum section.

Little intro before I start. My father has a website with drupal, and usually I help him with problems. But I encountered something now that I can’t solve alone. So I would like you people to help me out a bit.

I will write down everything what happened up until where we are now.

The website has a Contact menu and under it there is a form, with a mailto part (the form is sent to my father’s e-mail), and this form didn’t work, the site said that the form has been sent but nothing arrived to my father’s e-mail. I will skip the troubleshooting part to drupal. So I tought that maybe with some kind of miracle one of the modules that helps the form is not enabled. So I turned on PHP Filter first, because the mailto uses PHP coding and I tought that will help with the problem but instead it crashed and an error message popped in after pushing the “Send” button. I disabled PHP Filter and enabled Contact module but the same happened, same error message popped in. I could still disable the Contact module, but nothing changed the form still popped in the error message. I logged out from admin user and tried to login back (the site said that the form has been filled earlier and we could take a look at them and my father tought that that is the problem and I showed him that he is wrong), and when I tried to login, but the same error message popped in as when we tried to use the form. Without being logged in the error message not popping up at the form but still not sending the e-mail. I tried to clear the cache in the browser with all of the browser’s history but didn’t help. I even cleared the DNS cache on the computer using CMD, but still nothing happened. I tried to login on a different computer using the same username, but I still get the error message.

Here is a a screenshot of the error message, some of the site’s parts are visible.
Sorry I couldn’t make the default add picture work here.
This is in Hungarian I will write here what it says:
Hiba = Error
Hibaüzenet = Error message.
A webhelyen nem várt hiba történt. Később érdemes újra próbálni. = An unexpected error occured on the website. Try again later.

I can’t remember exactly which drupal version is on the site at the moment. If I remember right it’s 7.X.

Thank you for your help in advance.

Drupal version: