Excel stopped calculating automatically — formulas not updating
You change an input cell and the formula that depends on it doesn't move — not until you save the file, or press F9. That's not a bug in your formula. Excel's calculation mode for this workbook has been switched to Manual, and every dependent formula is frozen until you force a recalculation.
How to tell this is what's happening
Change a number in a clearly-referenced input cell — nothing recalculates immediately, though the status bar along the bottom may briefly show “Calculate” to indicate pending work. Open Formulas → Calculation Options and check whether Manual is selected instead of Automatic. If it is, you've found it.
Why it happens
Calculation mode isn't an Excel-wide setting — it's saved inside each workbook, and it applies per Excel session across every open workbook, not per file. If you open a large model that someone deliberately set to Manual (often done to stop huge files from recalculating on every keystroke), Excel switches your whole session to Manual, including files you had open before that were previously calculating fine.
Fixing it and forcing a recalculation
Switch back with Formulas → Calculation Options → Automatic. If values still look stale immediately after switching, it's because Automatic mode only recalculates cells Excel thinks have changed since the last calculation — press Ctrl+Alt+F9 to force a full recalculation of every formula in every open workbook, ignoring Excel's dependency cache. Use plain F9 for a normal recalc of all open workbooks, or Shift+F9 to recalculate only the active sheet.
Making sure it doesn't happen again
If you routinely receive files from someone else, check calculation mode right after opening any unfamiliar workbook, before you start relying on its numbers. Upload your file below to check its saved calculation mode and get a live recalculation of every formula, so you can see whether values were actually current before you go any further.