I've just updated my PC from a P3 to a P4 and the technician used the same case to install the P4. I plugged it at home and the CPU overheated so the pc automatically shut off. I bought another case for the P4 and I assembled it myself but now although the power supply switches on and the LED's are in place, the monitor won't switch on. I also replaced the graphics card, all in vain. What could be wrong?

i'd take it back to the tech and have him explain why the cpu overheated. perhaps it is damaged.

Do you mean that the monitor does not get power? That is what "switch on" means. Do any indicator lights ON THE MONITOR come on or not? If not, check your power cord and make sure it is good. If still no good, you have a bad monitor. If your computer powers up, fans come on, the power light on the mobo is lit, you hear the harddrive spin up, the system beeps and the ROM drive lights blink like it is booting, but your monitor is dark BUT the power light on the monitor is on, reset your BIOS back to default settings (just pull the battery for 15 minutes). Then try to reboot. If that doesn't do it, check the data cable to the monitor for bent pins. If still no joy, swap out the monitor and see if a different one will work. If it still does not work also, your AGP slot or mainboard itself may be damaged. Give a man a fish and you feed him for a day; Teach a man to fish and you feed him for a lifetime; Then industry pollutes the water and kills all the fish.