All I could get out of the user was "half the screen is grey".

I quickly figured out it was probably the taskbar, and I tried asking if it was the top or bottom half, or left or right. "Half" was the reply...

After a lot of attempts and little progress, I made a breakthrough by asking in which corner the clock was. "Bottom left", aha! And with that I managed to guide the user to dragging the taskbar back and resizing it properly.

Granted this was back in Windows 98 where the taskbar wasn't locked, so was rather easy to do, it was probably their first PC and they were non-native so language barrier.