think neighbor might have committed suicide?

I would probably call the police and fill them in on the situation.

I'd go knock on the door and see if anyone answers. If you're concerned something might have happened to him, I'd call the police department and say you're concerned about your neighbor, and explain he's had martial problems, there have been no lights on, and the car hasn't moved from the driveway. Let the police check it out. There's nothing wrong with worrying about a neighbor, and more people should have the concern you do. If it turns out he's fine, then you still did a good thing by having someone check and make sure.

Try calling the cops. They deal with this kind of stuff.

Edit: Not a very good idea to listen to wrecked. Your neighbor may be mentally unstable at the moment and if he's still alive under those conditions, you have no idea what he might be planning.

Most police departments will do whats called a welfare check. If you suspect something is wrong, call your local police and let them know you are concerned. If its nothing, at least youll get some peace of mind.

If it were me, and I were on good talking terms with him, I'd go over and knock, try to make contact. If nothing, call your local police dept and explain to them what you just explained here. They can send an officer to check stuff out.

I think you should call the local police (not 911 obviously) and just tell them your concerns. They deal with this kind of stuff all the time and have policies in place for dealing with such concerns!

look if the mailbox is empty. If it is, everything is under control. If not, ring and if there's is no response, I wouldn't recommend calling the police, but you could give a try. Maybe no one is in there.

call the police and inform them they will check it out