Search button on sidebar is not working

Hi,

I recently faced a search problem in my website when I tried to find an article by a word.
When I wrote any word in the search area, all of the articles have appeared, or something else, irrelevant to this word.
My website is in Greek language, however, I tried to search an English word with the same results.

(If you believe that the problem is another plugin, then, there is nothing to do because I need them all as they are.)

Could you please tell me, if anyone knows, what's the problem for such errors?

Thank you in advance,
Dimitra