I would tell them that I apologize for the long wait and ask how may i assist them?
I would be very nice, and not let there bad day or attitude get the better of me.
Greet them and apologize for the delay, listen to their complaint and attempt to work on their transaction at the teller window. Speak to them in a courteous manner when necessary.
I would give them positives to their negatives, it almost always works!