Onlay and root canal? (photos)

From the x-ray and pictures that you posted it appears that a root canal, buildup and crown will be necessary to restore your tooth at a minimum. Another option is an implant and a crown. The decay does appear to be very deep so the risk of a root canal failing is greater. An implant and crown will be a more permanent solution. 

Thank you for your question and photos.  From the the x-ray and photos, it appears that a root canal may be necessary before undergoing additional procedures to replace the missing portion of the tooth.  Even if you are not experiencing any pain, it is possible that decay can travel down to the nerve of the tooth and kill it before any pain occurs.  Please see a dentist and go over your options whether it be trying to save this tooth or opting to remove it and replacing it.  Good luck!