Traffic signal like the one on a street that give you permission to cross a intersection.
Signals down is a very generic terminology. Could be anything, I do believe the Michigan line is dispatch from Chicago so a break in the communication lines is quite possible. It can be as simple as a burnt out lamp. No lamp, must consider it red and stop and call dispatch. With PTC positive train control more computer and communication needs, easier to mess it up.
Why it takes to repair a signal down call 8 hours. Well you have to notice the problem, then call out your repair member(s). Then the MOW forces have to figure out what wrong, and where it when wrong. Then fix the problem, that might need parts, or completely replace something expensive that not found in there truck.
Just thinking here: Digging up a fiber optic line. Car accident that hit a control box and smashed it. Someone fail to pay a bill for a service.
Might do well to email Michigan DOT and ask them. A 8 hour delay on a 6 hour trip is not something we need to expect or accept.