computer will not bootup - what is wrong?

there are several reasons a computer wont boot . Lets start with power . Is it getting any ? does it post ? Do you here beeps when you start it or try to ? Do you here fans at all . If theres nothing at all it could be the power supply if it posts beeps you can tell if theres a problem by the beeps . They post in patterns 3long and a short or 3shorts or 2long and a single listen and tell us we can tell a lot from the beeps . If its actually running and the screen doesnt come on it could be a problem with the monitor . give us more information and we can better answer your question