If astronomers determine that it is destined to collapse, this means that in something like 20 billion years from now the expansion will have slowed to a stop and begun to reverse in a collapse phase that will last until about 50 billion years from now when the Big Crunch will happen. The universe will get much warmer from its 2.7 K today to billions of degrees within the last few minutes. Galaxies will be torn asunder about 1- 5 billion years before the Big Crunch, and the stars themselves will cease to exist within 1 million years before the Big Crunch. Stars will become the coolest objects in the universe about 1 million years before the Big Crunch.
If the universe is destined to expand forever, in about 10 trillion years, all of the stars will 'go out' with no new ones formed as the interstellar medium is eliminated and only degenerate stellar cinders are left. Beyond that era, nothing more can really be said because the times scales for anything new and interesting to happen then become of order 10^30 to 10^40 years when protons may decay away into electrons and neutrinos, and 10^60 years when stellar-mass black holes evaporate by the Hawking Process. Pretty bleak for life.