In reply to AnnaSpanna:
There are lots of ways for it to happen, but my experience with bruising of complete big and small toenails is:... The bruise under the nail is basically a blister - this is best drained (I go in from the front, under the white bit), which will leave the nail "hollow". Once it is hollow the skin underneath will start toughening up and forming into a new nail pretty quickly. You can leave the old nail on and it'll loosen up a bit, but it might never fall off without some manual assistance... The old nail can be cut back painlessly (it's not alive or attached to anything - use clippers and take off as much as you can. You'll notice that where the nail is attached at the sides it can be "peeled" off the skin quite easily and painlessly... The hardest bit to take off is the very back, but this also comes off when it's ready by peeling it forward from the cuticle - this is painless and simple, but always feels a bit gross...
A big toenail might take 6-8 months to grow back to near normal. Coincidentally this is just the right time between OMM and LAMM races, after which it will come off again and the cycle repeat...
Partial bruising is different and can lead to complications. One of my big toenails got a small bruise from winter climbing, never came off, but developed a secondary nail growing under the first. When the whole lot came off a few years later and grew back, it grew back wrong... The nail doesn't grow off the front of the toe like it should (it doesn't form a white fringe for cutting), but rather just bulges up and out... I can cut into the bulge and file it so it it roughly normal shaped, but the nail definitely isn't right... The best way to keep it in check is to lose it roughly once a year, and I have no idea what the long term prognosis is!...