Normally arrow keys move the cursor from one cell to the next. Suddenly, my arrow keys have stopped doing that: instead they move the whole spreadsheet up/down or left/right. Anyone know how to restore the normal arrow keys function?

It sounds like you've got something resting on the Ctrl button. Either that, or you've somehow managed to put the 'Ctrl Lock' on. Ctrl & Shift usually clears it.

Best solution is to reset the pc.

Sounds like you've got scroll lock on - you can tell with Excel as you'll have SCRL in the status bar at the bottom right.

Turn it off with your keyboard "SCRL LCK" key or bring up the on-screen keyboard (under the Accessories menu somewhere) and use that if you don't have a scroll lock key

Yep, I must have hit the scroll lock key by mistake. (Not that I even knew there was such a thing.) All sorted now.

Thanks very much