If you are trying to start up Part 2, and you are either
a) Unable to find the Part 2 launcher
Or
b) Unable to begin Part 2 once you have launched it
You need to check these things first:
a) If the launcher is unavailable, be sure you have downloaded and installed the Part 2 DLC. If Steam does not immediately recognize the DLC, restart Steam and it will refresh your library, making the game visible.
You will see this when you press play on the main game:
b) If you are unable to find your end-game save file from Part 1 when you press Continue on Part 2-
You have an end-game save file that was made before I put out a patch last year that caused a complication in that save file. I made a post about it back when I made the patch, but it has been a while and I can't expect everyone to be aware of it.
You have two options:
You can either replay Part 1 from the beginning all the way through to the end again.
Or
You can load an already existing save file from Part 1 and get to the end that way.
The point is- you will need an up to date end-game save file in order to access Part 2.