In my opinion, if the doors were in good working order when you or the other renters moved in, and the slider on the bottom is brocken due to neglect or abuse, then the renters are responsible. Unfortunately if this is the case, then good luck finding a roomate to admit they did it. Perhaps it was broke without anyone's knowlege.
On the other hand, if the slider came off because the screws backed off, then the landlord should fix it. More often then not, these types of closet doors are troublesome.
Approach the landlord as a freind and ask what can be done about a door that does not work that you may not have broken. Ask him/her if it is preferred to leave it broken, and leave it to them when they want to fix it.
If the landlord is not knowlegeble on home maintenance/repairs, then perhaps he just never really fixed the real problem, and just put the door back on the tracks. They must have a slider at the bottom and the top rollers should be properly adjusted.
Maybe if the landlord does'nt want to deal with it, maybe you can find a freind to help you, or get advise from a knowlegable salesperson at a hardware store, chances are that the sliders are the main problem, and they're cheap. Hope this helps.