Unable to enter anything into a cell
Solution 1:
It's not that you needed to close another workbook; all you need to do is to click on a different workbook, and then you can then come back and edit the cell in your original document. Just taking the focus away from the first document frees it up.
We can reproduce this at will, but we don't know why or what makes it happen to get stuck in the first place. Others on the web have this issue as well. So far, only 1 of our users have this issue and we just put everyone onto 2013.