Had a root canal, crown fell off. Now there is decay and I need surgery. What should I do?

Sorry to hear that your tooth didn't last as long as you hoped for.  Sometimes these things happen with root canal teeth depending on how compromised the tooth was to begin with.  It is likely you will need an extraction and dental implant.  Best of luck.  

This is unfortunate, but something which can happen following a root canal treatment. You may need to get the tooth extracted and have a dental implant placed.

Unfortunately, decay can occur even around a crown with a perfect
margin. They really need to be checked every six months since once decay gets
under a crown it can destroy the tooth, sometimes without pain. Today, dental
implants really are the standard for replacing missing teeth. The benefits
(preserving bone, no cutting down of adjacent teeth) make implants the first
choice if you've lost a tooth.

I am so sorry that your root canal treated tooth only lasted 5 years.  The goal of this treatment, and the subsequent placement of the crown, is so that you may keep it for a lifetime.  Research has shown an 86-93% success rate for 10 years.  Unfortunately, as long as you have teeth, you can get decay.  The risk of decay, especially around restorations like crowns and fillings, just gets worse as we get older due to the gums receding.  Proper oral hygiene and regular maintenance visits with your dentist can minimize this risk.  Many times, though, decay that starts at the margin and extends under the crown of a  root canal treated tooth can be difficult to detect due to lack of sensitivity in the tooth (the nerve is no longer present) and x-rays that are blocked due to metal in the crown.  However the decay happened, now you are faced with a situation of needing to replace the tooth.  A dental implant is the best option for a good long term prognosis in many cases.  I hope you will see a practitioner who can best evaluate your situation and explain to you the pros and cons of each option. It is also never a bad idea to get a second opinion.  Good luck!