Restore partition from merged partition
You'd have to do some very serious data forensics on the partition and even then, you're not guaranteed to get all your data back, as some of it will likely have been overwritten already. If it's important, you're probably best off paying for a professional data recovery company to get the data back. If it's not important, just reinstall bootcamp from scratch.

You could try to do this yourself, though you'd risk making things worse, so don't do this unless you don't really care about losing all the data. Basically, you would need to re-shrink your OSX partition to its original size, create the bootcamp partition in the exact same position and size (e.g. "80GB" is not good enough - this needs to be precise down to the sector) as before without initialising/formatting it (just adding the partition table entry - NOT via Disk Utility). Then, you can run an NTFS data rescue utility on this partition and see what it finds. You probably don't know the exact size and start of the old bootcamp partition, so you either need to see if it's recorded in any log or backed up files, otherwise you'll have to scan the drive for the NTFS header, if it's still there. Again, there are tools that can automate this.

I can't overstate how easy this is to mess up though, so unless you're prepared to lose the whole lot forever, let a professional handle it.