I had 4 root canals done in 2011 because of a car accident. 1 of the 4 broke off 2014 and now another 1 feels very loose and hurt really bad. why?

Several possibilitie: there can be many causes and without x Rays and exam it's impossible to diagnose in this format. I recommend seeing an Endodontist ASAP

Vague: Need a thorough history, x-rays and an examination to determine the diagnosis and prognosis of your problems. Don't hesitate. See a dentist asap.

Possibilities: Possibilities include failed root canal treatment, fracture, additional trauma, gum disease, failure to return for regular 6-month post-treatment reevaluations, failure of post-treatment restorations, etc. You will have to be examined by a Dentist, and the sooner the better.