Broken menu on theme after updates

Hi I updated the Catch Everest theme on my site http://www.lynnswillow.co.uk/ to the latest version 3.5.2 and Wordpress to version 5.6 and now the menu seems to have gone wrong, it's displaying like a mobile menu stacked vertically down the screen and not horizontally across as it should. Can anyone please help? This is a live site and it now looks terrible!! Thanks Rebecca