Hello, as a preface; the calculate option in Tools is set on automatic

I built a large spreadsheet six weeks ago that has worked perfectly until
today. the sheets that contain VLookup formulas that reference data in
adjacent sheets.

The cells with the VLookup's won't claculate unless i place the cursor into
the cell (as if to edit the formula) and hit the enter key. then the formula
works, but only that cell. F9 does nothing. Restarting does nothing.

Does anyone have any ideas?