Can you squeeze the wood back together without cracking it some place else? Sounds like the wood has dried out. If it has, you're probably going to be shoveling against the tide. The wood might just keep splitting until it is in two pieces. Whatever you do, be careful. If the wood is dry and brittle, the attempt to repair it might make it worse. Without looking at it, we can't really tell.
Maybe, just maybe, if the wood isn't too badly dried, you can put it in a humidity chamber for a while and slowly raise the moisture content of the wood back to normal levels. The wood will swell. The split will go back together enough to be clamped together. You can use marine glue (AKA: "Gorilla Glue") to mend the split. Also, consider using a mending plate on the hidden side of the wood if you can.
I've not done this myself but I've seen others do it. The process can take weeks or months, even. But, if done properly, you'd never know it was ever broken.