Excel pointer won't work...

....sorry if this has been answered....

Apparently I've toggled something and the cross-hair pointer won't select cells any more--no double clicking, nothing. It still moves around the screen, changes to an arrow at the top of the column etc. but doesn't do anything.

I can move around individual cells with the up/down/left/right arrows, so all isn't lost, but it's just annoying not knowing how to un-toggle the pointer.