Has anyone ever had this happen and what do you do to prevent it from happening?

I have my scene and lights all set up then press render

the box states "loading texture part" and then "loading objects" and then

"rendering shadow map" 

and when I return an hour later it still says "rendering shadow map" and nothing is happening
so I come back an hour later and it still states:

"Rendering shadow map"

WTF?

what causes this and what can prevent it from occurring?