One common problem that people have with Dell monitors is if the monitor starts showing “Dell Auto Detect Analog Input” before entering power saving mode, even if the monitor is connected to a computer.
This happens when the monitor is not getting a signal. Typically, this either means that the computer it is connected to has entered power saving mode, is hibernating, or has been shut down. Sometimes simply restarting the computer using the power button is enough to wake it up. If not, make sure that your monitor cable is properly connected to the port in the back of the computer and that the cable is secure.
If you are still unable to get the computer to show anything on the screen, then you will need to diagnose the issue with the computer. Listen for beeps, and if you hear multiple beeps when you switch the computer on note down whether the beeps are long or short and what order. Search for beep code troubleshooting tables and see if you can find something that matches yours.
Sometimes the issue is as simple as something else that is plugged in to the computer malfunctioning. Remove all of the USB devices connected to the computer, except for the mouse and keyboard. Remove the power cable as well, and then press and hold the power button on the computer for ten seconds. Release the power button, and connect the power cable back to the computer. Switch on your computer.
If your computer still does not work, look at the front of the computer. There should be some numbers lit up near the front USB ports. Those numbers are troubleshooting codes and you may be able to get help if you search for them online. If you are confident working with computers, try opening the case and reseating the RAM and graphics card. If you are not confident, ask a technician to do this for you.