If your raft was previously moving, and just stopped moving as you loaded in, you can simply fix this by exiting to the menu and reloading the world.
If your raft is still not moving you can try the following: Face the sail away from the wind (so the wind is blowing into the back of the sail) - You can find out which way the wind is facing with a streamer.
If the raft still does not move you can also replace your sail by picking it up. Also, make sure to raise any anchors you have dropped.
My raft isn't moving!
