More info would be nice. At the moment it could be the psu, a short, etc. Is the video on board or a separate card? Remove everthing from the case and place on a non conductive table. Install only one stick of ram, video card if not built in, cpu with thermal paste, heat sink and fan. Plug in the psu to the
mobo, cpu fan, video card, etc. Plug in the monitor and make sure it is turned on (does happen). Use a small screwdriver to connect the power pins and see if the unit posts. If it does then you can put it all back in the case making sure there is no shorts between the
mobo and the case.