2000 MPV Won't Start

Ok I had a bad code which came up OX sensor. I had it read at AZ. Its been about a month and I haven't changed it yet . Forgot which one it was but they were expensive and the car still ran well. Just went out and went to start it and it turned over but just made an attempt to start ( 2 seconds) which it has done before many times.. I turned the key back and it turned over but sounded weird and absolutely no attempt by the engine to fire at all. Like no spark-or something.

Could this be because of the ox sensor not being replaced ??
It also has a bad IAC valve that I have been meaning to change.

Anyone have a course of action I should try in order to eliminate whats wrong.
One of the worst things I have found is the car just not starting for no good reason.
I don't want to jump around and and just start testing things -Maybe someone has had a similar problem and could help me.
I used to be a car mechanic in the 80s but now with all the sensor junk and computers its a little over my head.
Why couldn't it be the brakes or something !!

Thanks in advance -I'll check back later. Crys