My temporary crown just fell out, it is in the back of my mouth #2 . Do I have to get it replaced or can I leave the crown off?

If the tooth had a root canal you will not be in any pain with it off so in that sense there is no rush. But it is best to have it placed back on as soon as you can. A good tip is to use fixodent (denture adhesive) to place it back in yourself if you can. Sometimes the orientation makes it tricky but most times it it is easy to place back in yourself until you can see your dentist.

It would be best to have the temp crown recemented as soon as possible to ensure that the permanent crown fits when it comes back from the laboratory.  I would call your dentist and they should make time to see you 

It is best to recement the temporary crown as it is a space holder for the permanent crown.  Call your dentist and see what they recommend.  I'm sure they will take care of you.