2000 Volvo 660 Model Detroit Engine

So I was going down the highway bobtail and all of a sudden the truck just started acting as if it was in neutral, so I took it out of the gear a put it back in gear still acting like it was in neutral, so I let it roll off to the side of the road, came to a complete stop ,then put it in first, it went right into gear but as I ease off the clutch I get nothing the truck won't even move a little bit, it just acts as if it's in neutral, I already had brand new clutches put in not long ago, anybody knows what this could be?

 

No sir no grinding at all just goes in gear without a sound

 

The mechanic trying to tell me it's the transmission,the same guy who did the clutches

 

I only drove the truck twice since the clutches was done and both times bobtail and no longer than 40 miles

 

And when you say restart meaning cranking?