Moen handshower head #128875 how do you take it apart to fix leak in head and put back together?

SOURCE: leaky handshower hansgrohe croma c100

Grab a wrench and give it at least a quarter-turn. Hand tight isn't enough with a shower, even with teflon tape.

SOURCE: Moen shower head drips. Single lever shower only.

Probably need to remove and replace the cartridge. Often sediment will tear part of the o-ring or seal on the cartridge and cause this problem...
varies by model, but generally, remove the cap on the knob, unscrew the knob, remove the keyed sleeve, remove the round trim plate & shut off the service stops (verify they are fully closed by pulling out on the valve stem) next remove the retaining clip (needle nose and be gentle) that holds the cartridge in place, then pull the cartridge straight out with vice grips or similar (or get a puller). clean the cartridge, flush out the valve body by slightly opening both service stops, reinstall cartridge and clip, open service stops and see if it still leaks. if it does, take your cartridge to the store (any reasonable HW or plumbing store should have a replacement) and get a new one. then reverse the process to reinstall.
If you send me the model I'll get you more specific instrucitons and pictures...

SOURCE: the cartrige inside my moen faucet leaks

moens website. they have parts break down and diagrams

SOURCE: Moen T2301 tub faucet

you may have deposit's,(mineral) in water way's. maybe piece of cartridge came off and blocking water. try removing cart' and inspect brass valve body,to see if any thing is there. check cartridge as well.

SOURCE: Leaking shower head

Very few shower heads are made to be rebuilt and any O-ring will always be proprietory (i.e. specific to that brand and part). Rohl has an excellent customer service department. The can tell you if the parts are available and will also tell you to get them. 1-800-777-9762.