Excel - lost number keys

Excel 11.6.6 (Mac OS 10.5.8)
All of a sudden the other day, my number keys stopped working in Excel. Only the "surrogate" direction arrow number keys do anything, and that is to behave just like the arrow keys. (2, 4, 8, and 6) The other number keys do nothing. I've searched for a setting in preferences, played with the Insert key and num lock, but nothing revives the old behavior: getting a number when a number key is pressed. What am I missing here?

What kind of keyboard do you use? Maybe the keyboard is broken?

I use a Matias keyboard. This only happens when in Excel. The number keys work fine in Calculator, for example. It's a USB Mac/Windows keyboard. I think the keyboard is fine. I suspect there is a keyboard shortcut that I accidently invoked.