help please

I've been making sourdough up till this warm weather decended upon us with good results. However last week I made some in the same old way and left it to prove for the usual 16-18 hours, but when i went to bake it the following day I removed the couche which covers the baskets, and saw it had really ballooned up to about 4 times it's usual size at that stage. So I turned it out to slash it and it collapsed, needless to say it didn't rise in the oven and came out like a brick. Any ideas as to why? Was it because the weather being a bit warmer meant that it got too gassy???