Loose tooth post-root canal & crowning, how can this be remedied?

Loose from? If the crown is loose, intact and no caries, the crown might be able to be re-cemented. If the structure inside the crown is decayed or fractured, at least a new post and crown. If the tooth is unrestorable due to root fracture, periodontal breakdown, untreatable infection, etc. an extraction would be in order. This needs to be evaluated in person by a dentist who can discuss your options.
Evaluation. The cause of the mobility needs to be determined... bone loss? Fracture? Periodontal problems? Post loose? Only a local dentist who can examine you can find the cause. Give the office a call.
See your dentist. asap to see the cause then they can determine the best course of treatment.
Complex. The root canal treatment is probably now recontaminated and usually will need to be 'retreated'. However, if this loosening has occurred, then it may be that there is not enough solid tooth to support the crown and biting forces. Might not be a good idea to fix this, but rather extract it and have an implant placed with a crown after 3 - 4 months. See prosthodontist for options. Good luck.