Win 98SE Machine will not start windows
 

I have a win98SE Machine that been working fine. I needed to do the annual
cleanup, so I opened the machine up [which I've done many times before,
without any issues]. However, after I put everthing back together, I wanted
the start the unit, so I pressed the power button, all I get is the CD / DVD
drives light, and the floppy light, and I hear a loud power transformer hum.
But nothing else happens. Nothing comes up on the monitor, and I have to hard
power the unit down! I triple checked the connections and I just cannot
figure this out.
Can you please help?