Unable to Enter Text Into Excel 2013 Cells

All of a sudden, for no reason, I am unable to enter any kind of text (dates, numbers, or plain text) into any of the cells in my SS. It worked fine when I first created it several weeks ago, but no more. When I select a cell and attempt to enter text, it appears in the field immediately above the body of the SS (the one where you go to correct or edit already existing text without changing the rest of the text in the cell - I don't know what you call it). But the cursor itself just moves along in the cell and no text appears. I have tried various solutions I saw on the Internet (turning off the computer, moving the cursor with the arrow key from the cell to the right, into the cell into which I want to enter the text, going into Options, etc. - nothing works so far.

Any ideas from anyone would be most welcome.