When a foundation starts to crack and experience structural issues, it will begin to separate at the cracks. This puts stress on your home's frame and starts to cause cracks in your walls, ceilings and floors. It can begin to cause instability in the frame and can potentially cause part of your home to collapse.
As your foundation begins to collapse, any utilities installed in, under or around your foundation are in danger. Electrical wires or water pipes run under and through your foundation are in danger of getting severed. Your sewage system, whether you have a septic tank or not, can also get severed or blocked. As the foundation problems cause your home's structure to become unstable, that can put pressure on the pipes and wires running throughout your home. This can cause water damage which can lead to mold and create a fire hazard.
A shift in your foundation due to structural problems can cause major problems with your windows and doorways. The first indication that you may have a problem is that you begin to notice that all of your doors and windows do not move smoothly in their frames. Over time it can become difficult to open and close windows and doors. Eventually you will begin to see cracks in the windows and doors that can allow heat to escape and raise your energy bills.
Foundation problems that are allowed to persist will eventually lead to a house that is uninhabitable. Not only will the deformation of door and window frames make it it difficult to secure the home and maintain environmental control, but the structural instability will eventually make it unsafe to enter the home at all.