Help! Two year old crown came off!?

Okay,so for whatever reason, I never got the permanent crown put on my tooth..in the back..molar I believe.After a lot of pain there a dentist could not pinpoint(I kept saying it was the area under the crown),the crown fell off. I now am in possession of the perm. crown,from the dentist ( I have moved to another state). My question is that after looking at the tooth under the temp. crown, there are several areas that look brownish...decay? The temp. was on for two years with no problems. What determines whether or not you need a root c or extraction? Do you always need a crown as well as an root c? Thank you for any help. yes,I was stupid to leave it on so long.