I made a post awhile back about the same sort of thing happening to me.

After last season I hung my bow on the rack down in my basement (easily 10 degrees cooler than upstairs) and drew it at least once a day every day. The first really warm day of the year I decided I wanted to shoot. I took my bow upstairs, set it on my desk and started loading the targets, bow boxes, etc into the truck to leave. When I walked back into my room to put the bow in it's case, I noticed the string had snapped directly on the upper cam underneith the serving material.

I have yet to find out what caused the break. Whether it was poor string material or perhaps something sharp between the cam and the string I have yet to find out, but I know for sure that it wasn't heat that caused mine to snap.

Depending on where your's snapped, heat may have been the leading factor. There are many things that could have caused it, just be thankful it didn't happen when you went to draw it back!