2003 envoy not start

So my boyfriend is having troubles. He can't start
the envoy. It was running and working fine. He was
installing a new stero) and he noticed that the
gages weren't working at all. And now there's no
power windows and no power locks and it won't
start. When he goes to start it the lights on the
dash come on and the car lights come on but it
won't start. What would be the issue of this. Please
help. He's checked the battery it's fully charged and
all the wires are connected