What's wrong with it? Need more information please.
What is wrong with the door?
What is the problem with the door?
If the glass is broken, then you’re going need to replace the whole door, because getting a piece of custom tempered glass is going to be expensive. It must be tempered, or be laminated safety glass.
If the gasket is leaking around the frame, you can probably caulk it was some silicone caulk, after you clean any water deposits off.
Silicone is not forgiving, so take your time.
If the hinge has failed, most commodity grade shower doors are aluminum. Aluminum is soft and any kind of steel fasteners that screw into aluminum must be stainless steel, or you will have galvanic reaction, which will eat big holes in the aluminum.
Hi Enzo: If we knew exactly what the problem is, we could be more helpful. Thanks