I have an exposed root canal. what should i do?

Go see your dentist: Not sure if you mean that you have had root canal done and now that the temporary is loose or you have a big cavity and it is so deep that the root canal space become exposed (which is quite painful). In either case, cover it up with a temporary filling that you can get from a drug store and go to your dentist right the way. Any contamination trapped underneath the temporary needs to be cleaned.

Call dentist: You need to call your dentist and have him evaluate the tooth and root canal to give you all your treatment options. It's possible that the root canal might need to be retreated prior to any restorative procedures (fillings, crown) can be done. It's been shown in studies that bacteria can migrate through the root canal to the tip of the root within days of exposure of root canal filling.