I would like to answer your query in two parts. Firstly you said you have gap in the crown and gums. l would recommend you to get the replacement for the ill fitting crown. Secondly you said you had undergone root canal treatment of the tooth. If treated properly and there is no periapical infection you won't have pain in that specific tooth. There might be cavities in formed in the adjacent teeth due to food lodgement and pain might be due to decay.
Next Steps
Refer to a specialist center, they will take an X-ray and find the tooth involved and do the root canal treatment of the involved tooth. For replacement of crown refer to a Prosthodontist.
Health Tips
use warm saline gargle to maintain a debris free oral environment till you get further treatment done.