broke a crown from the gumline. What should I do? (photo)

It looks to me that you had a root canal done in that tooth and decay developed under the crown, resulting in the loss of the crown. Visit your dentist for an examination to reevaluate your options of trying to save the tooth, or removing it and placing a dental implant. In my honest opinion, I would remove the tooth and place an implant.

A clinical exam and gum probing would reveal the relationship between the fracture and bone level. Depending on the findings, it can be decided if the tooth is restorable. A crown lengthening procedure can also be done when indicated. 

Your best option is to see a dentist as soon as possible. That
tooth might be able to be saved, it could need a root canal or it could need to
be extracted. The only way to know is to see a dentist for an exam and an
x-ray. Don’t put this off, if it’s already hurting, it’s not going to get
better without treatment.

Definitely need to see the dentist to evaluate, but it appears the tooth is not salvageable.If this is the case, your options with the missing space after extraction is to do an implant, bridge or partial.Sincerely,Dr. Reiser

It is impossible without an xray to really advise you with this but see a good dentist ASAP--they will be able to determine if the tooth can be saved and offer you your options. It may be hopeless if the decay extends close to the jaw bone. Good luck to you