Does the tooth have to be cleaned before putting adhesive on to attach a crown that has fallen off?

That is a great question.  This depends on the type of crown that you have and how it is being cemented.  All of the crowns that I place are bonded in, therefore there is a few steps that must occur before a great bond can be obtained.  1.  The tooth is etched with a phosphoric acid material.2. Bonding agent is placed, then light cured.  3.  Resin cement is placed inside the restoration, placed on the tooth, them light cured. 4. Cement cleaned, polished and xray taken.  This is protocol for our office, but there are different ways to do it.  Good luck.  

There
can be many reasons a crown continues to come off. It could be because of the
bite or because the prepared tooth is very short, not necessarily because of
how it was cemented. Crown cements are not all the same and the technique will
vary depending on the type of crown and type of cement. Over drying a tooth
before using certain cements can actually cause sensitivity. The inside of a
crown should be cleaned of old cement before re-cementing, however.