Dell computer won't come out of standby mode?

all of the sudden while i was online, my computer shut off. i tried rebooting it but the yellow light just stays on and nothing happens. no fan, doesn't start up regularly, it just stays as if it were on standby mode. ive tried unlplugging everything but nothing happens--the yellow light just reappears. if you could give any input on how to fix this please share! should i leave it off and try turning it on later? thanks for your help