Masks are occasionally dislodged when divers are congregating on the anchor line, doing safety stops in a bait bundle, and someone's wayward fin kicks someone else below them in the face.
Rarely is the mask entirely knocked off your face, but it can nevertheless be disconcerting.
Generally speaking, you grab it before it leaves your head entirely, reposition it, and clear it.
No worries.
If it's kicked off entirely, hopefully you have a buddy who can snab it for you before it drops too far. If yes, no worries - reposition it and clear it. If no, basically it isn't a problem unless you're wearing contacts, in which case you might lose a contact.
Most divers can see reasonably well without their masks, assuming they open their eyes. That is to say, given some ambient light, if your mask gets kicked off at 20' you can see other divers, you can see objects, deal with your ascent, etc. Mask-less divers may need some help during the rest of the ascent if you lose your mask down at depth, but its rarely a huge deal - particularly if you have your buddy there to assist you. Try it on your next dive in shallow water. Remove your mask and run it up your arm or hand it to your buddy. Try to spend the next 5-10 minutes conducting your dive calmly, just cruising around in shallow water. It isn't that challenging.
If you're penetrating a wreck or a cave, where the loss of a mask may compromise your ability to see the line or function as a team-member, most divers doing these dives carry a spare mask.
But for general open water recreational diving, carrying a spare mask or using a clip to tether your mask to your BC is overkill. Pay attention to whats going on above you during the ascent at the end of your dives, and become comfortable without a mask on so you don't freak out if it gets kicked off. Then just relax... The likelihood of you losing it to begin with is slim, and if you do, it still isn't likely to be much of a problem.
Regards,
Doc