one question.

i mining eth, then in a week ago i change to mine ZEC, just to try it for and hours, but then i swich back to ETH mining but i could not start miner (before i tried mining ZEC everthink went fine). so i reinstal GPU driver and then i try again ETH and it works and i leave it till today. Today i want to try ZEC, but could not start the miner.

What can be the problem?