Breaking the no decompression limit (NDL) of your dive is a potentially serious situation. If this were to happen, you would be entering an extremely advanced diving situation that can safely be done only with very specific prior planning, special gases, and a complex dive management plan.
Dives beyond the NDL would require you to make staged decompression stops on your way back to the surface to help avoid the threat of decompression-related injuries. The best way to avoid this is to plan conservatively, watch your time and depth, and use a PDC. A PDC will constantly update your decompression status and display safe dive times and warnings if you are approaching your limits.
One of the most serious consequences of unplanned decompression diving is that you may run low on breathing gas while you are performing the required stops. Occasionally, a dive boat will have a spare cylinder and regulator, commonly referred to as a “hang cylinder,” hanging over the side of the boat to provide extra gas for this situation. However, this should not be taken as an open invitation to abuse limits. Stay well within the NDL on all dives.