Formula shows 0 instead of the right answer — circular reference
The formula isn't broken — it's stuck. A cell somewhere in this chain refers back to itself, directly or through a few other cells, and Excel's response to that unsolvable loop is to display 0 rather than an answer.
How to find the actual loop
Excel usually shows a Circular Reference warning dialog when you first create the loop, and lists the cell under Formulas → Error Checking → Circular References afterward — but that menu only shows the cell where Excel detected the loop, not necessarily every cell involved in it. Click into that cell and use Formulas → Trace Precedents to follow the chain backward until you find where it doubles back on itself.
Why it's showing 0 specifically
Without iterative calculation enabled (File → Options → Formulas → “Enable iterative calculation” unchecked, which is Excel's default), a circular formula simply can't be evaluated, and Excel substitutes 0 as a placeholder rather than leaving the cell blank. If iterative calculation is enabled but Maximum Iterations is set very low (the default is 100) or Maximum Change is too tight, the calculation can also settle at 0 without actually converging on the value you intended.
Two different fixes, for two different intents
If the circular reference is a mistake — a formula that should reference a different cell but accidentally includes its own — fix the reference and the loop disappears. If the circularity is intentional (some financial models legitimately need a self-referencing calculation, like interest that depends on a balance that includes prior interest), turn on iterative calculation and raise Maximum Iterations until the value stabilizes rather than resetting to 0 each time.
Checking your file
Upload it below to see every formula and its live recomputed result — a formula stuck at 0 due to an unresolved circular reference will show up clearly next to what it should be evaluating.