Ceramic towel holder was pulled out of the dry wall. The dry wall was cut out to fit holder into it. Hole has no backing so you see straight to the dry wall on the other side. Looks like thin set still on back of holder. What is the best way to reinstall holder when there appears to be nothing to attach it to?

No tile around it? Cut out a 2" x 32" piece of drywall and add pieces of wood (2x3 or 2x4) between 2 sets of studs, angling screws into the studs and making sure they are driven in flush or past flush. Patch the drywall, compound, paint.

Attach the towel bar with the proper method for how it's designed. If it doesn't have a slot on the back for a hidden bracket that is attached to the wall, you'll have to use thinset again. Clean the old stuff off.

As you now know Ewfiii, ceramic towel holders are not meant to be installed into drywall....meant to be installed into ceramic tile!!

One old-fashioned way to fix this would be to crumple up a bunch of newspaper kinda soaked into some thinset mortar..pushing it into the wall so it bunches up and stays in place in behind the hole (fiberglass insulation stuffed in and put just below the hole will help here). Then, remove the old thinset from the ceramic towel holder and force new thinset mortar into the HOLE in the back of the ceramic towel bar holder and build this up high/tall....push into the hole so the thinset of the ceramic towel bar holder meets the thinset mortar of the newspaper (now is also the time to install the towel bar). Tape the ceramic towel bar holder in place
and wait 24 hours...should hold pretty good if you spread the newspaper wide enough that it acts as an anchor behind the drywall once dried...;)

Otherwise, there really isn't a great way to install this as it will eventually fall out when pushed down on hard enough. Patching the wall as Joypulv suggested will not be beneficial here...will eventually pull off the wall again!

Try the method mentioned and see how you make out. My guess is that it will work fine, but the other holder will eventually pop, too and you'll need to do this again!!

If all else fails, you would patch the wall and purchase a NEW WALL MOUNTED towel bar and anchor the towel bar holder into studs or use toggle bolts...never have to worry about towel bars again!!

Good luck!

Mark