Shower Door Handle Broken
Shower door handles can receive a lot of abuse. Not only do they get pulled and pushed, but they also have to survive in wet, humid conditions. Occasionally they become loose or break.
Whether your door handle is on a swinging or sliding door, they will be held to the door frame by screws on the frame of the shower door or the glass itself if frameless. In most cases, the handle will become loose and you can simply tighten it with a screwdriver.
Door handles on a swinging door are small and typically have 2 screws holding it on, while sliding shower doors have a longer handle that spans the width of the door. There will be a handle on each door, with the outside handle doubling as a towel bar.
Handles don't usually break, but rather just need to be tightened when you first notice them becoming loose. In many cases, once the handle becomes neglected, the screws will corrode and break rather than the handle breaking. So you can replace the screw and be done with it.
If the handle does actually break, you will likely need to order a new one from a plumbing supplier. Big box stores don't usually carry them because they don't usually break. Bring in the broken handle and order the right one, and then it is a simple matter of just screwing it in.