Re: extreme decay under crown!
I think that if the tooth is so decayed then there is nothing to work with so you can't just remake the crown. You could extract the tooth and have the dentist make a bridge, or you can get an implant. The implant will be more expensive but the bridge won't be terribly LESS expensive, and with that you are also messing with the tooth in front of and behind it, where with the implant it just affects that one tooth area.
If there was an opening somewhere around the crown, then certainly it can begin to decay. Sorry you have to go through this.