First, the camshaft stops moving and all the timing is lost. The pistons then hit the valves and damage both parts. Depending on the force with which the pistons and valves hit, you could also end up with decimated cylinder walls. The broken timing belt basically destroys this entire part of the engine.
There are some engine components that can break without causing serious damage. You can go to your mechanic and have the part replaced without problems. However, this is not the case with a toothed belt. When the timing belt breaks, the crankshaft will continue to rotate, while the camshaft will stop turning.
As a result, the pistons will continue to rise and fall in their cylinders and, in some cases, may hit the valves. When this happens, pistons, valves, and cylinder heads are damaged, and because it happens so quickly, the damage is virtually instantaneous. The timing belt is held in place by a series of nuts. However, over time, these can loosen up.
When this happens, oil can leak around the timing belt cover. As you can imagine, this can be a serious problem. When the oil flow is interrupted, there is a great risk that the engine will overheat. If this happens, it can result in significant damage and a large repair bill.