Yesterday I've had some freezing problem, so I re-connected ram, hdd etc. Now the problem is I can't turn it on .I am sure that I haven't made any loose connection. So what might the problem be?
Checking for Loose Connections
The most likely reason for the freezing problem with your computer is a loose connection. So, even though you have already looked into this, you should take a good look at each and every connection. It is possible that something has gotten loose without being readily visible. Check power cords, cables, peripherals, etc. While you are at it, make sure that the electrical outlet is working if you have the computer plugged into it. Take the RAM out and reseat it just to be sure. Another step you should take is to clean the computer and remove any dust, especially from the fan.
Booting up in Safe Mode when Pc Won't Start
Since the PC won't start on its own, you should see if you can get it to boot up in Safe Mode. This strategy should work if all of the connections are in place. Turn the power on for the PC, press the "F8" on the keyboard several times until the computer starts to boot up. Depending on the type of PC you have, you might need to use an alternate key on the keyboard such as "F2", "F12," or "F10" when booting up. Once you get to the options, select "Last Known Good Configuration."
While you probably already did this, make sure that the RAM that you installed is the correct type. If it isn't, that would explain why you are having these issues.
Resolving Startup error by Booting up with the Installation Disk
If you have it available, try booting up using the installation disk that came with your computer. Insert the disk as soon as you turn the power on, press the "ESC" key on the keyboard. Once you see the words "Check System," press the "F1" key so that your computer boots up from the installation disk. Select the option to "repair your operating system."