How will I know if a new crown is needed or if the one that came off (i was able to save) can be re-applied?

Typically if the crown still fits properly, there isn't any open areas at the edge of the crown (called open crown margins) or decay or breakage on the tooth, a crown can be recemented.  However, if there has been some breakage of it or it no longer fits tightly, then you would better off have your new dentist make you a new crown.Whether or not it can be still be recemented is not something a dentist could answer for you without examining your tooth, your crown and the way your crown fits onto your tooth.  I would not be disappointed with the 35+ good years the old crown has served you.  Very few things today last forever and you use your teeth everyday....it was a pretty good investment that lasted you a long time.

35 years is a long time for a crown to last, especially with a post! I applaud your previous dentist's work. It sounds like it was very well done!In an ideal world, a dental crown would be a "forever" treatment that never needs maintenance or re-fabrication. There are multiple reasons why the crown might have fallen off, especially after so many years-- even if there is no pain involved. Since you have a post, it's a safe assumption that your tooth has a root canal treatment, so really, you shouldn't be able to feel anything, since the nerves were removed as part of the procedure. That being said, you may have recurrent cavities or other issues that may have predisposed you to having the crown come off. I wouldn't be surprised if you need a new crown in that case.The alternative option is that the cement that kept your crown stuck to the tooth has simply worn away with time. The fact that you bit into something hard with it may have applied enough force to finally take it out. If that's the case, there's a possibility that the crown can be salvaged.I hope that clears things up a bit! :)