We had an issue today of not being able to view the worksheet tabs across the bottom of a Google Sheet spreadsheet. This one was extremely annoying as I couldn’t see the screen of the user in question. Oddly, when I opened up the same sheet, I couldn’t either. But simply scrolling down brought the tabs into view. Didn’t work for the person I was helping, though. So, I poked around the internets and found this discussion in Google Docs Help. There are a couple of things they recommend.
if that works, then we need to disable extensions until we determine the one causing the problem
Anyway, I hope you find this helpful. Let me know in the comments.