Hi,
I can see that when I use the search field there, I get redirected to the menu item with the alias "apotelesmata-anazitisis".
In the settings of your filter module, you'll find a "menu" setting where you can enter the id of the menu item that will be used for the filtering done by the module.
So either that field is empty or has a wrong value, and thus HikaShop randomly automatically select that menu item, or you've entered the id of that menu item there.
Then, in your filters module that you have on the left sidebar, you must have configured the "menu assignment" setting to not display the left sidebar filter module on that "apotelesmata-anazitisis" menu item.
What to do exactly depends on how you want it to work.
You could just edit the left sidebar filter module to add the "apotelesmata-anazitisis" menu item to the "menu assignment" setting for example. Or you could change the "menu" setting of the search filter module to use another menu item which displays all your products and already has the left sidebar filters module assigned to it.