If you want to switch between cells using the cursor keys, or if you want to type in a cell, Excel will not allow it. If you restore all open windows that are open in maximized window mode (ie remove them from maximizing, not minimized), then Excel will be able to operate normally again.
The problem occurs on two different computers with Windows 10 (private) and Windows 7 (office). The problem occurs in Excel 2016, now Office 365. The problem may have been in Excel 2010 as well.
Additional environment conditions: Extended Desktop. Several Excel folders opened; all maximized. The error is reproducible, but occurs intermittently (under not yet comprehensible conditions).