When you say lost, do you mean the entire tip or did it get degloved? Tails have the misfortune of being dragged through all of their bedding, urine, etc, so I'd probably take her in to have the affected area removed if it's been degloved. If it's just totally gone and the stump doesn't look inflamed and isn't weeping pus or anything it'll probably be ok. Whatever you do, keep it clean with saline and do bedding changes as often as possible. I tend to move mine to a tub with paper towels if they've got an injury that way I can change bedding daily and disinfect easily.
Her already had a nail on the tip which was around 2 ish milimeters, so the part that she had lost now was around 2-3 milimeters at most, I don't have a picture, but it looks an feels like a dry wound, looks like dry blood and she doesn't complain when I touch or hold it. It went from the tip being white to looking dry. Not sure how much would be the entire tip, but I wouldn't say it's the whole thing, just 2-3 milimeters on top of the old spike she had on her tail.
I don't know, it's always had a spike, well, all of my rats have a tiny spike (half milimeter ish, well, not all of them, but i've seen some of them having it), but she had the biggest one, haven't seen her doing anything with it, but in this case I think it was her cagemate playing rough, but for something like that, it's still very tiny, so I'm not sure on what caused it..
If all of your rats have this then I'd recommend getting a humidifier and supplementing their diet with fatty acids. What kind of bedding do you use? My only other concern is that it may be fungal. I'm not all that familiar with tail ailments though.
This is the picture that comes up when I look for "tail nail" on google http://s20.photobucket.com/user/Sharlibby/media/Finstail.jpg.html?t=1176738488
Some of mine down have it, not sure on how many exactly, but it might just be two that have it or 3, I have 5 rats, 2 cages.
Hemp bedding is what I use, they don't itch allot and the rats don't have dry tails, just really soft tails.
I've read on one site, might of been someone here, that it has with leadership to do.
Allot of people, both in Sweden where I am and on this forum, and other rat forums appears to have the same tail spike, it appears to be normal according to allot of people to have a spike at the end.