I can think of a number of reasons off the top of my head.
- clean the cpu cooler. (doubt this is the problem, but in case you haven't done it, do it.
- clean the video card cooler (monitor:"no signal")or replace the video card or try reinstalling video card drivers (may require reinstalling windows from scratch)
- if you can purchase a $15 pci video card from ebay and it works, it's your video. see below about optical drive light first.
- video cable or power cable is loose. try reconnecting. also try plugging in the monitor at all points. may have been kicked loose. it's happened before! inexpensive fix, too! [just noticed you have tried different monitors]
- dead monitor. solution: try different monitor. lcd/led monitors only last so long. a friend of mine's only lasted the period of the warranty: 3 years. ahh, just noticed you already tried that.
- the cd drive should not open by itself when the system starts. that would be abnormal behavior. normally it would just blink and if a disc is inside, the head would move back and forth inside while windows and the BIOS/UEFI checks for existence a [bootable] disc, then then the light would go out.
- it could be the cpu, IF the optical drive light does not come on at any time or if system does not beep, or beeps incessantly (look up the beep codes in motherboard manual). hard to tell if it's the motherboard or the cpu.
- could be a virus/trojan/worm. someone I know had something that did the same thing to their system. power light was on in front of computer, but would not completely power up. installed linux mint 13 kde and all was fine.
- try burning and booting memtest86+ and set computer to boot from cd if that's possible, as a test. might come up with video. go through the memory test and see if it comes up with anything. note that it won't be touching the hard disk, so chances are it might work if it's a virus.