Thanks Will1988, I have noticed the tension is not great even when mower is engaged, which led to the thought of it needed adjusting. Hard to say which pulley the belt came off as I had to put it back on both of the middle pulleys, not the 2 outer ones with the covers on them. The belt was actually outside of what looks like belt guards to keep the belt in place on the pulleys. I was mowing at the highest height(4")and was mowing grass that was 1 to 2 ft high. Once I was able to get the belt back on the pulleys I continued with the mowing and did not have any more issues. I thought that perhaps the high grass had become tangled in the pulleys and caused the belt to slide off of them.
Thanks