Fixing a stone retaining wall?

Question

Some of the stones have fallen and dirt is creeping down the wall from the top.

Answer 1

Ideally, you remove the stones above the missing ones and rebuild the wall with mortar to set the stones together.

This is a lot of work. You could also dig out any infiltrated dirt, replace the stone, and mortar around it. Either way, you have to fill the gap the stone left, or the dirt will keep on washing away.