Do you have any experience writing a story at least 10-20k + and then making a sequel to it after basically putting the characters on a shelf for almost a year?
No, but the first thing I'd do is go back and read the story before I decided on doing anything with them.
How do you get to know your own characters again after putting them aside for close to a year?
I'd doodle them and look back on any old drawings/writing I had with them before, then doodle/write/ just think about them and run some scenarios through my head
My story ends on a sort of cliff hanger.. I have no IDEA how to start this next "book" so to speak XD Have you ever dealt with something similar?
Sort of. The best thing I think you can do is write an outline, just to get some ideas out, you may only keep a couple of scenes from the outline, but hopefully it'll get your creative juices flowing.