Excel not responding to arrow keys

Yesterday afternoon (27 September 2021), I noticed that Excel spreadsheets are no longer responding to my computer's arrow keys. The only way to shift to a different cell is scroll and click. I also can't select multiple cells.

I'm having the same problems with both new and existing spreadsheets. I didn't have the problems in the morning. One of my housemates had the same problems.

So, what's going on?

Is this change part of an update, or is there some other problem going on? I use spreadsheets a lot, so this is a major pain.

And how do I solve it?

Thanks.