You wake up one morning, turn on the TV, and there is a sci-fi film about people panicking because our Solar System is about to be hit by another star. You hate such movies. You change channels, but surprisingly, there is the same story on other channels as well. It was not fiction at all. It was happening for real. A hypervelocity brown dwarf star about 13 times as massive as Jupiter, approaching at 4% of the speed of light, has only been noticed because such suns can be even at room temperature, and some don't shine much. It will touch our Sun as it will fly by our system in just a few hours.
This violent side collision would shave off a bit of our star. The immense amounts of powerfully ejected matter would blast through the whole Solar System and furiously crash into planets. If the doomed Earth survived at all, it would be ruthlessly resurfaced and changed into liquid magma in which our ruined cities would submerge. The surface would cool again only millions of years later. This would spell the end of our forsaken civilization and possibly even life on our planet. Counterintuitively, the reduction of the mass of our Sun would extend its life by hundreds of millions to billions of years and make the orbits of the surviving planets expand because of the lower gravitational grip on them. Less massive stars have longer lifespans significantly. The blasted matter would eventually settle into a disk in which new planets could form as new neighbors to the ones that survived this surreal armageddon.
And surreal it would be, literally, because it would not be likely to happen. The distances between stars in our part of the galaxy are just too vast. A star touching another star system in our location in the Universe means something else. It would just be an approach that might disrupt some asteroids in the Oort Cloud, resulting in some comet strikes on planets. Infinitely rarely, stars can approach each other somewhat closer. This can result in their planetary systems becoming unstable. Some worlds can change orbits, collide, fall into one of the stars and be destroyed, or some can be ejected and become rogue planets.
Stars occasionally collide directly in some dense clusters of stars and in the galaxy's central bulge. There is evidence of it because blue straggler stars have been detected in these locations. They formed via the collisions of stars. They are brighter, and their light is bluer than similarly massive stars that formed in a usual way. If suns collide at a very high speed, they can dissipate and become gas clouds. If they hit each other at an angle, they might start revolving around their common center of mass, the barycenter, ever closer until even billions of years later when they might collide. Such impacts can expel massive amounts of matter, forming a protoplanetary disk where new worlds can coalesce.