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 you have the Ask toolbar in your browser, remove it
try opening the spreadsheet using "incognito mode"
try resizing the browser screen
try using Firefox (or another browser. If you're in Firefox, then try Chrome or IE)
Anyway, I hope you find this helpful. Let me know in the comments.
if that works, then we need to disable extensions until we determine the one causing the problem