The best thing to do if you bump them off the roost is wait right there, unless you hear another gobbler or have another good spot I would move. This is coming from a guy that has never had much land or much good turkey hunting land, I am used to not having other options.
I don't know why, but they seem to come back to their roost once they have calmed down. If it was a gobbler and some hens, he should be gobbling his head off after he settles down.
There is no guaruntees in turkey huntin though!