1995 nissan sentra xe 1.6 cyl, 5 speed

hello, I have a 1995 nissan sentra xe, 1.6 5 speed if that matters. 2 weeks ago I was backing up in my driveway into the street then stalled. Then I tried starting it back up and it wouldn’t start. My boyfriends dad found out that it was the timing chain so we bought both the upper and lower chain brand new and his dad is in the middle of replacing it at the moment and he ran into a problem? he said the camshaft won’t turn as he’s trying to replace it. What is the problem? What needs to be done? someone please help me!!