There is a building just south of Wattz Consumer Electronics with a roof you can access by fire escape. It's a bit strange: the fire escape has a set of stairs that go up, then you drop onto a balcony (it's a pretty tight fit), then climb another set of stairs to get to the top. I climbed up with MacCready. He got up fine, but apparently he can't find his way down. I was doing the Silver Shroud mission at the time and didn't realize he was stuck there, I assumed he just disappeared (which has happened from time to time with other companions, though they usually reappear eventually). I finished the quest and ended up switching companions (Hancock) and it asked me to send MacCready somewhere, so I sent him to Greentop Nursery. Carried on with my game, thought nothing of it. Eventually I decided I wanted to get MacCready, but I couldn't find him at Greentop. I checked all my settlements with no luck, so I thought maybe if I start playing his quest for a bit I'll come across him, and luckily I did, on that roof. I thought maybe he just needed some guidance down, so without thinking, I stupidly swapped him out with my current companion (Codsworth) and tried to send him down, with no luck. So now he and Codsworth are both stuck up there. I've tried commanding them to move somewhere on the ground (they just say they can't get there), tried sending them to a settlement (didn't work), and tried blasting them off the roof with a missle launcher (nothing).

Does anyone have any idea how I can solve this?

Turns out I can knock them off the building with a melee weapon, if anyone runs into this issue.