Computer Will not boot

Ok, so Im nearly finished with my first build, and I have everything I need to at leaat get the computer to boot, and when I power it up it seems as if it is turning on, everything all the fans and stuff turn on and the HDD whirrs, and the heatsinks warm up, but there doesen't seem to be any video signal. I tried using both the onboard video, and a cheap PCIE card and still got nothing. Im only using a 250w PSU at the moment so I thought that might be the problem so I tried booting with nothing but the mobo, processor and RAM. Still nothing. It seems like it's booting up in every way except no video. I even got a new mobo but that didn't work either. The mobo company isn't any help, I have moved jumpers tried all combinations of connections and devices, I tried a different PSU and different monitor. I have no idea what the problem is. I have put time and money into this and now when it all come together it's a bust. This is very disappointing, and I would appreciate some help