To replace a door panel is easy, and inexpensive (most like a bathroom door is not wood). However if you want to fix it, here is some tips found in google. If you don't have the tools and materials, it might cost as much as repalcing the door though.
Hole in Hollow-Core Door
If you have a hollow-core door with a serious hole in it, consider replacing the entire door; hollow-core doors are modestly priced. You can repair a hole if you're willing to paint the door to hide the patch. To do this, you'll need a can of spray foam insulation.
1) Fill the hole with the foam.
2) Let it dry and expand overnight.
3) Use a razor blade to slice off the excess, cutting the mound just slightly lower than the door's surface.
4) With a 3-inch putty knife, apply vinyl spackling compound to the patch, and draw it smooth to the door's surface. Allow to dry.
5) Sand lightly with fine sandpaper, then touch up with paint.