If a mask gets knocked off, try to remember that as long as you are breathing, you are okay!
It's good to practice relevant skills in the pool so you get comfortable dealing with a dislodged mask. In your course you will certainly learn to remove/replace/clear your mask. In the case that it goes to the bottom, that gets a little trickier. I've never lost a mask, but I have had mine kicked partly off my face in a crowded dive site. I've learned to stay out of other divers' fins since then!
A drill that I did recently at a workshop was to remove the mask and swim around with my buddy guiding me, including going to deeper/shallower parts of the pool and doing bouyancy adjustments without the mask. This was done with eyes closed, but in real life you might keep them open to help orient you. It was great for increasing comfort for being without a mask.
If you loose a mask and it's sitting on the bottom in not too deep water, you may be able to retrieve it with your buddies' help. Otherwise, you'll want to call the dive and surface with the help of your buddy.
Many divers carry a small backup mask in a BC pocket, so that is an option too.