Crank, which I have talked about before.
Crank: High Voltage came out in 2009, was directed by Mark Neveldine and Brian Taylor and stars Jason Statham and Amy Smart. (IMDB page)
I thought I was Dead
This movie start exactly where the previous one left off, with the main character discovering that he was not as dead as he expected.
This is something that you can spring on your players as well. In a situation when they should have died, they might discover that they survive, but for somebody else’s purposes.
Stealing the Heart
The whole reason why the main character is being kept alive is because somebody else wants his heart as a transplant, since he has proven its quality by surviving for so long in the previous movie.
You can easily do the same thing to the PCs, with somebody being after them for their internal organs, or possibly, in a more supernatural setting, perhaps their soul. Maybe in the setting it is possible to gain somebody’s strength by eating their heart/brain/liver?
Device to keep alive
Without his heart, the character ends up with an artificial one, and needs a battery pack to keep it going.
Perhaps a PC could have something that she needs to keep her going? A battery pack for a droid PC? Or even a magic item of some sort that is holding the characters death at bay?
There is no reason why an NPC could not have one of those. Perhaps a villain in a Superhero game has a stasis device that keeps him in this dimension, and will revert to his home dimension if that device is destroyed.
This movies is filled with characters from the previous one, as the main character runs into them with the flimsiest of pretexts.
The fact is, if the players like a PC, they will not care how you put him back into the story. He just happens to be having a beer at the bar the characters walk into, even if the last time they saw him was getting trampled by wildebeest on another planet. I am sure the players will forgive you.
This is just another way to reintroduce a character, perhaps with a twist. IF the NPC died without a doubt, feel free to return him as his twin/clone/body double.
Of course, you could always do a switch: “That was not me, that was my twin brother, he offered to take care of a couple of things for me while I was out of town”
You can even do this with PCs, but watch out for going overboard (unless you feel like it)