Computer screen is black when turned on?

recently i used a vacumn cleaning to suck out all the old dust on my motherboard after i plugged everything back in but now when i press on button it flashes green to orange and doesnt load up, i can hear the fan blowing though,the screen is black i tryed pulling out the memory then putting it back in but still nothing is happening, any idea what the problem could be, the computer is quite old DELL GX 260 series, please help thanks :)