What makes vlookup come up as n/a?

Hey, Im currently in college and had an assessment in excel the other day. We were given an excel file with questions, one of which was to do a vlookup. (can usually do these in my sleep). Every time i tried to do it on the day it kept coming up n/a. I renamed the table array as per usual rules but it still wouldnt work. Not one person in the class could get it to work and it caused bedlam because the lecturer kept saying it was our fault. Later on that day we were told there was a problem with the file itself and that the vlookup wouldnt be graded. They wouldnt tell us what the problem was, does anyone know what could cause this to happen? Trying to find out because it affected the whole exam as other questions were based off the vlookup. need info for the appeal. Thank you!!