Retaining walls must have weep holes in them. The weep holes are there to allow moisture and water that accumulates behind the wall. If the weep holes are not installed water and moisture will accumulate behind the wall. Soil behind the retaining wall that is wet will expand when it freezes. The frozen soil will expand and push the wall forward.
If one were to fill an ice cube tray with dry sand and put that ice cube tray in the freeze nothing will happen. In fact the dry soil may not even freeze absent at least some moisture. If one were to put soaking wet soil in the ice cube trays when the wet soil freezes it will pop out of the trays.
Wet and or damp soil is the problem behind the retaining wall. Wet or damp soil is problematic because the wall will move and fail. Prior to failure of the wall that lacks weep holes one could excavate behind the wall, install porous fill such as gravel then install weep holes.
After the retaining wall starts to move forward and to fail there is not much one can do other than remove and replace the wall. Removal and replacement of the wall will be very expensive. The cost to replace a four foot high retaining wall can be about $200 or so per running foot. If the wall is 100 feet long the cost to remove and replace the wall can be $20,000 WOW.
Should the wall be higher than four feet a design usually must be prepared by a Licensed Professional Engineer at an additional cost. Often a property survey must be prepared by a Licensed Surveyor at an additional cost. Nothing is cheap. Wall replacement can be very expensive.
Be aware of home sellers who want to put in the sales contract the retaining wall is in “as is condition”. To me “as is condition” does not mean falling down or about to fall down. Check with your attorney to determine what your their opinion is.