Left my bike to dry after a wet and gritty ride. Now the BB is very stiff and doesnt turn freely.

Its happened in the past, but given a few turns its loosened up, but that doesnt seem to be working this time.

Any suggestions

There's been some ingress of water/silt/etc
This has rusted the balls, and or set.

If your BB is doing this, long term, it's shot.

Short term. Get the bike on either side and give the bearings a massive dose of GT85 or the like. Take the chain off the rings. Gently rock the cranks back and forth until you get them to turn. After that try giving them a dose of some thinker lubricant. Chain lube might do.

Good luck.

SB