Why will my Pathfinder turn over but won't start?

I went to go to work one morning and my 99
Nissan Pathfinder wud turn over but wud not start.
It acted like it was not firing. I changed the whole
distributor, which includes the ignition control
module and the coil. So I checked everything and it
is getting fire and fuel, and still won't start. I also
changed the crankshaft sensor cause someone
said that may be the problem, and nothing. Same
thing just turns over but won't start. Shud I change
the timing belt next? Would that cause it not to
start, even tho it was running perfect when I parked
it last? Please help, can't afford to keep getting it
wrong. Plugs and plug wires are good also.