My mower suddenly stopped moving while mowing and I found a six inch long spring on the ground after moving it. It seems there is no tension on the dricve belt now. Can anyone tell me if this spring is the problem and if so where is goes on the mower drive belt tensioners?

Sounds like that's your issue.  I suspect a fairly heavy spring(maybe 1" in diameter)?
 
They'll be one idler that moves back and forth to keep tension on the belt. Connected 
to that, likely a moving (back and forth) with a hole in it for that spring.  
 
There should be another hole(stationary), nearby where the idler pulley would go to keep spring tension on the belt.
 
Here's the parts diagram, but it's hard to make out what goes where:
 
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/husqvarna/tractors-ride-mowers/yth23v48-96043017500-2013-09/drive 
 
 
.