If you signed up for iTunes Match, it may finish with Step 1:
Step 1. Gathering information about your iTunes library.
and then eventually get stuck at Step 2:
Step 2. Matching your music with songs in the iTunes Store.
If you do get stuck, try going ahead and pressing the “Stop” button at the bottom of the iTunes Match screen, and then pressing the iTunes March “Start” button once it is available.
It may take a bit, and it will probably repeat Step 1, but once it gets to Step 2 it should start from where it previously got stuck.
Some people report having to do this repeatedly, but for me just once did the trick!