Can you find the check in AP-R?
 
F2 by check # only and see if it is there.
If so what is the date?
 
If not do AP-R by the Vendor only...
...dont use check#s....a #accidentally entered wrong will fall out of order
...dont use dates ....(we ntered one a yr off  one time)
...dont use checking accts....its too easy to accidentally pick the wrong one.  
 
See if you can find a payment to that vendor for the amt in question  (with out using dates or accts or #s as a varible)
If not....check to see if you could have accidentally picked the wrong vendor.    
 Do you have the back & front of the cancelled check?