Check the status bar for a circular reference warning
Go to the Formulas tab
Click Error Checking
Select Circular References
Review the listed cell references
Click each reference to jump to the circular cell
Use Trace Precedents to see which cells feed into the formula
Use Trace Dependents to see which cells rely on the formula
Inspect formulas for self-references or indirect loops
Use Evaluate Formula to step through the calculation
Repeat until all circular references are removed