There are several possible options as to what could’ve happened. 

One of them is that the truck is stuck somewhere along the road. In this case, you can simply restart the game to reload all trucks so they could continue working as usual.

Another option is better explained with a screenshot below:
 

Traffic jams indeed happen sometimes. You need to close your game with a force stop and open it (maybe several times).

Also, if you see only some of your trucks on the roads, it is possible that the other trucks are just stuck somewhere. In this case, you can restart the game as well and see if all trucks appear in the game afterwards.