Take note that at least on PC, your companion CAN DIE. so there are times where if you don't see them, they might be dead. If this is the case, use the "move to (ref ID) command to get to them and the go back into the console, use your mouse to click on your companion and type "resurrect" this will bring them back to life. be careful of this. if you gave them anything to carry for you, it will be lost so you might want to loot them first.
i love my cheathappens trainers but there is a command that will keep your companion from dying. setessential (reference ID) 1
Sadly the companion AI is not as well done in this game as expected. So sometimes they can get stuck or even that matter of them dieing. But sometimes if there just stuck or got lost because you bolted off. They usually come back to there original spots in a few days. Thats if they where not killed.