Computer Turns On But Displays nothing. Case's LED light is off.

Hello,
Something like 4 weeks ago I have shut down my computer at night. When I woke up and tried to turn it on again, the pc has turned on - all the fans were spinning and everything looked alright, but nothing was shown on my screen and the blue LED light on my case (CM ELITE311) was off. After I pressed the power button for 30 seconds to "clear" the electricity from the computer I have managed to boot it up again.

From this time it became worse and worse. 2 weeks ago I discovered I have to unplug the computer from the power and all the cables when I turn it off so I will be able to boot it up next time. But now even this does not help. I tried to switch the ram slot, replace the cmos battery but it is still not booting up.

Everytime I managed to boot up the pc I saw the blue led light. And everytime I couldn't the light was off.

Thank you in advanced!