Remote Access/ blue screen

Hi, my sister has my old 2011 mac air computer and she and my mum have installed norton’s security on their computers. recently my sister was having trouble with her norton’s and so she contacted them and they said they needed to remotely access the computer and uninstall and reinstall norton’s so they did that and as soon as the norton’s person connected to the computer the screensaver changed automatically from a picture she had to a blank blue screen which she didn’t think anything of as she assumed that just signified that he had access now and that it would change back automatically when he disconnected. however when the norton’s stuff was all finished and they disconnected and ended the conversation etc the blue screen remained. does anyone know why that would be? nothing else seems to have been changed and it was an automatic switch from the photo to the blue screen when he connected, we assumed it would automatically change back once everything was finished. We are quite a technological illiterate family and are quite confused about this and what would have caused it and how we go about fixing it, is it as simple as changing the picture back again. thankyou for your help :)