How can I reattach a ceramic towel bar end bracket to a ceramic tile wall?
It appears to me that the towel bar may have been originally attached using tile grout. It should have been installed with tile mortar, and could have been grouted after the mortar was set. Mortar creates a much stronger bond.

Considering that you probably don't have mortar on hand, I'd swing by your local tile store and ask what they have in a small squeeze tube that would work. Otherwise, construction adhesive, pure silicone, or other household adhesives rated for moist areas would do as well. 

You'll then want to caulk or grout around the base to help keep water out and create a finished appearance.