VLOOKUP & Data Validation
 

I have 2 VLOOKUP formulas in one cell that are nested within an IF function. The lookup value is a cell that has a data validation box in it, and the data validation has a fairly "complex" formula in it. The vlookup formula was working but now it suddenly isn't. I thought it may be because I entered a more complicated formula in my data validation cell, but even if I delete the data validation & type in a value the VLOOKUP no longer works. I'm at a loss as to what to do to fix it.

I am new to VLOOKUP & MATCH functions, so I know I must be missing something. You can see named ranges in the formulas below. Please help!

Here are my formulas:

Formula in the data validation cell: =OFFSET(DeptStart,MATCH($C$76,DeptColumn,0)-1,1,COUNTIF(DeptColumn,$C$76),1)

Formula in the VLOOKUP cell: =IFERROR(IF($G$5="Yes",VLOOKUP($F$76,Instructor,4, FALSE),VLOOKUP($F$76,Instructor,3,FALSE)),0)

Here is my spreadsheet (data changed for protection)