A #REF! error means a formula is pointing to a cell, range, row, column or sheet reference that no longer exists. It often happens after deleting rows or columns, moving data, or removing a worksheet.
Start with the obvious checks
Find the cell with the error and click into the formula bar. Look for #REF! inside the formula. Replace it with the correct cell or range reference. If you recently deleted data, undoing the deletion may restore the original formula reference. Check copied formulas too: a single broken reference can spread through an entire table.
What else to check
If the formula worked before, compare it with a neighbouring working formula. Check whether a referenced worksheet was renamed or deleted, and make sure external workbook links still point to the right file.
When to get help
Send the file or explain what happened and get help repairing broken formulas.