Please go to Theme Options > Header > Search > and make sure that search functionality is enabled.
If so, try to disable 3rd party plugins to check if one of the plugins causes the problem on your site.
Yes, of course, SEARCH is enabled. In fact, I have found out that the only page the Search is working (the search field displayed after clicking on the search symbol) is SAMPLE PAGE. On other pages it does not work (the search field is not displayed). It was working fine prior to the theme upgrade.
Which plugins I should try disable? I’m not sure which plugins belong to the theme, so I should not disable them? Please help.
Also, is it possible that the Custom CSS modified earlier by your support team (in order to fix other problems/tickets logged) could cause this problem and all other display not functioning problems (MENU on mobile not working, FILTER by price / attributes not working, ENTER PROMO CODE not working, QUICK VIEW not working)?
We were able to figure out what is causing ALL these problems. Somehow Rev Slider was embedding into a specific menu item – see this link `http://test.olgalishansky.com/wp-content/uploads/2018/12/Screen-Shot-2018-12-17-at-11.37.46-AM.png
This is internal theme bug, which needs to be fixed. If you need steps to reproduce it, please let us know.