my computer cannot boot after connecting two two graphics PCI cards to a single CPU, why?

Answer:

their drivers are conflict against each other, gotta straighten that out. If it won't boot you probably shorted your motherboard out, generate sure they are secure within their slots! Why do you need 2 illustrative cards? 1 good one will do any opening you need.
Check your contact points,. Suggest to verbs them and reconnect.
Maybe they are not matched to your cpu or mainboard.
You could possibly be consuming too much power from the power supply depending on what type of graphics cards you are using. I would say verbs one out. Honestly why do you need two?
You can't connect two PCI graphics cards to a single computer. They hold to be SLI compatible, and only PCI Express cards can be SLI compatible. So cart one out and you should be fine.