2002 Honda Civic Timing belt change

My mechanic replaced my timing belt and water pump at 107000. It ran fine. About 30 days and 1400 miles later, the car wouldn't start. At first I heard a clicking noise then just a spinning sound under the hood. I took it back to them and they said that when they took the timing belt cover off, the belt was still tight but the camshaft gear was 90 degrees off from the other gear which was at TDC. They're telling me that they think it was the tensioner, which they are going to replace but they are not sure. Again, the belt was tight but the gear was 90 degrees off of TDC and it ran fine for 1400 miles. Any ideas?