Excel Wont Allow Data Entry into Cells

When I open an existing excel workbook and try to add data to my existing tables I simply cannot do it. I will type in the digits and press "enter" or "tab" and it disappears. There is no conditional formatting, the text colour is not white, no data validation, nothing of the sort. I can drag existing data down or across but cannot subsequently edit that data. I have no idea what's going on. The same error is occurring on all existing workbooks, but not on new work books. When I press enter or tab, the highlighted cell doesn't move down or to the right.

They are not opened as Read-Only, and there are no "Permission Settings" as far as I know. The first few workbooks I opened today did not have this issue. I'm quite perplexed by this, and I'll need to figure it out quickly, as most of my work day is data entry.