What happens to the lunar modules in space?

Most lunar modules eventually crashed back into moon once they weren't needed anymore. They were separated from the Command/Service Module and left in lunar orbit. This orbit would decay and the ascent stage would impact the moon's surface. Some of the seismology sensors set up on the moon's surface were able to record this impact.

The lunar module from Apollo 13 is in the Pacific Ocean. Because the lunar modules have a radiation source, the one in the Pacific Ocean is monitored to see if it is causing any contamination. Apollo 9's lunar module never left low Earth orbit, it presumably burnt up in Earth's atmosphere.

One of the Lunar Modules is actually in orbit around the Sun. For some reason, Apollo 10's ascent stage was sent on a course that would take it out of lunar orbit. It is still out there today in what is called an "extremely chaotic orbit."