The dark area on the tooth by the gumline looks like decay. Depending on the extent of the decay the treatment can be as simple as replacing the crown or as extensive as extraction of the tooth, grafting the bone and placing an implant. An x-ray would be the first step to start planning the treatment.
From your description and the radiograph ( though not clear) it looks like tooth #4 has lost a lot of bone and doesn't have enough root support to hold itself in place and it's carrying the load of another tooth through the bridge. A consult with a periodontist ( gum and bone specialist) is in order to determine if grafting the bone could save the tooth or replacement is best. Redoing the bridge is not enough to stop the inflammation and bone loss.
From your description and the photograph it looks like the tooth might not be salvageable. Your best action would be to replace that tooth. If you are away and can't get to a dentist to at least remove the tooth and graft the bone, try to keep the area clean by brushing more frequently and rinsing with mouth wash to avoid infection.
In a situation like this you want something to protect the tooth from further breaking or moving. A new crown is needed if you can see a dentist were you are now ask them to make a temporary crown for the tooth. And when you get back to your regular dentist make a new crown and maybe discuss why this one broke off. It could be the material or the design of the crown or the bite.
It sounds like the tooth is non restorable at this point. If the root canal treatment and crown were done and the tooth starts to hurt again. The possibilities are a crack in the tooth or an infection. And in both situations the best outcome would be to remove the tooth and replace it.
