If you have been transferring data, make sure the value being looked up is in the same format as the data in the table.
For example, if you are looking up a serial number, say 12345, which is a number in the VLOOKUP formula BUT the 12345 is formatted as text in the table, the formula will return a false. That is, the lookup will not find the 12345.
Yeah - I've come across this before when copying data out of Access into Excel - what looks like a number is in fact text.
Solution:Insert a column next to your lookup values, then enter in the formula
=Value(A1) (assuming the list starts in A1)
Now copy this all the way down.
Next - copy this list of results and paste
VALUES into column A
Finally, delete column B
If this is the problem, the lookup will now work.