I would have conversation with your dentist and explain why you were unable to get back in to have the crown seated and that you have paid for it and would like to work out a way to get it finished. It sounds like the dentist may not have the crown any longer, so a new impression would...
Yes, a crown can last 30 years depending on many factors including how clean you keep it, your biting habits, the type of material it was made out of, the skill of the dentist and lab,etc. If you do not take care of your crown, you can get decay at the edges of it that then can work its way up...
If the xray reveals a well done root canal, then consider that the crown is the problem. Either a core build up around the post or a new post may be needed for better retention. Seek another opinion but bring the xrays.
