Pc is working but monitor says no signal detected

Hey i have a problem with my computer i switched it off 4days ago and wen i switch it on the monitor says no signal detected.the computer is properly working because it also gives me the same beep when i switch it on,i also tried another monitor to check if i have a monitor problem but it also showed the same. I dont know what might have gone wrong because i also opened it today to check if everything thing is plugged and in good condition and it seems to be ok with every component please help am using a Desktop P3 yesterday i tried to switch it on nd it only display for few seconds and say no signal detected