What happens to that mass once it goes into the hole and why can't we see it anymore?
It gets a little tricky.
So, first of all, I have to put this out there: we don't know 100% how black holes work, and even what they are. To this point, we know that singularities (black holes) are places where physics and math break down. They are points where HUGE amounts of matter ( >8 M☉(Solar Masses)) are condensed into a infinitely small point!
Now, with some GARGANTUAN stars (which can be upwards forty times the mass of the sun), you have basically infinite mass being condensed into infinity small points! What happens to the mass? We don't know beyond this point.
What happens in general? You have a black hole, something with so much mass in so little a space (or no space depending on how you look at it) that nothing can escape its gravitational pull after it passes the blackholes event horizon. Not even light.
Why don't we see it? Because the black hole's gravity pulls the light back so we can't see it! (Whether or not we would see something [the singularity] that is basically invisible is up to you to decide [because can you see something that is condensed infinity?) In essence, vision is light hitting the inside of our eyes, and our minds interpreting it!