Is it necessary to shut off the water in order to tighten a handle on a Moen faucet?
If all you're trying to do is tighten a loose handle - the answer is no - you don't have to shut off the water. The screw to the handle should be on top of the handle. Sometime it's under a cap which is usually just pressed in to the handle. Pry it off with a blade of some type and tighten the screw.
That should do it unless you have a setup that's totally different than most Moen faucet handles. In that case - send a picture.