Are you getting any beeps while your computer starts up or when it has already started? If yes, then refer to the following HP Support page to get details about different types of beeps and the respective issue they indicate:
If you are not getting any beeps, you can start troubleshooting the issue by first checking the connectivity and performance of your monitor. To do this, refer to the steps below:
1)Make sure that your monitor has power and the power light on it turns On when power is supplied to the monitor. If the light stays Off, turn Off the power supply- unplug and plug the power cord- reseat the power adaptor connection on your monitor's end- turn On the power supply. If the monitor's light glows, see if the computer still shows the blank screen. If yes, follow the steps ahead.
2)If your monitor is of flat panel LCD type, unplug its power cord and wait for around 30 seconds so that the electronics in your monitor are reset. Now reconnect the cord and turn On the monitor.
3)If you are still facing the blank screen issue, disconnect all the connected devices to your computer except mouse, keyboard, and monitor. Also remove memory cards from card reader slots (if any) and thumb drives connected to your computer (if any).
4)If the above steps do not resolve your issue, you can try (if possible) to connect your monitor to another video cable (if it is detachable) for testing purpose.
If the issue still persists, you can refer to the HP Support links below to troubleshoot your issue: