They can, and they will. Fusion will stop once all the elements have been converted to iron. Many black holes will form, protons will probably decay, black holes will evaporate, and nothing but smooth radiation will remain.

It's called the heat death of the universe.

https://en.wikipedia.org/wiki/Timeline_of_the_far_future

This is assuming the cosmic concordance model describes reality. There are other models where some sort of reverse expansion will occur. They call it either Big Crunch or Big Bounce.