After boting monitior is getting shut down
When the monitor shuts down, is the activity light on the monitor blinking yellow? or staying yellow constant? Have you tested another monitor? When does the monitor shutdown exactly what is the computer doing at the time the monitor shuts down?
1. Blinking yellow means failing monitor. Not all monitors will exhibit this behavior. For example my Acer Monitor blinks with a blue light when no video input is detected. It doesn't always mean the monitor is bad, but may indicate the video card is failing. Also check and/or replace the video cables
2. Solid yellow means no video input detected Not all monitors will exhibit this behavior. For example my Acer Monitor blinks with a blue light when no video input is detected. It doesn't always mean the monitor is bad, but may indicate the video card is failing. Also check and/or replace the video cables
3. Testing another monitor tells you if the video card is failing. If another monitor works, the problem is your old monitor failing, if a new monitor doesn't work, then the problem is more than likely a failing video card.
4. It is rare but sometimes the Video PCI slots can go bad, while another may work on the same motherboard. This further indicates if it's a motherboard issue, and not the monitor or the video card. Just change the PCI slots the video card is plugged in to and see if it works.
5. Power issues can cause this video issue as well; For example if your PSU is not strong enough to support your video card, it may exhibit no input detected issues past a certain point in the boot process. This is due to lack of power being supplied to the video card. MOST video cards have a warning of sorts if this is a case, but some do not. This issue is more common with higher end gaming video cards and/or lower end power supplies.
The best way to test this is to disconnect ALL devices from the motherboard that require power; For example; CD drives, Hard drives, USB devices that may be plugged in other than keyboards/mice. Only boot the PC with the video card, RAM, and CPU installed. If you get video start attaching 1 device at a time and doing a fresh boot with each device connected. So if the PC boots fine with video on the monitor when the CD Drive is plugged in... it's not the CD Drive causing it. Then hook up the hard drive WITH the CD drive, and see if the PC boots with video then... If it does not, you can infer a power issue might be causing the problem.
Another issue that may effect the video card directly pertaining to power issues are; Try swapping the power cables to the video card with a different power cable. That is to say a power cable coming from the Power Supply that isn't connected to anything or in use by anything else. It is possible a power cable line is bad and/or not working in which case it's a Power Supply issue.
In some cases; This test will be negative on all accounts; At this point, It may be your RAM/CPU causing the issue; Try reseating the ram, that is to say uninstall the ram physically, use a cloth to clean the gold prongs on the ram sticks, and place the ram sticks back into the motherboard; See if it boots up with video then. If it does not; Try removing ram sticks 1 or 2 at a time as required and checking again. This helps determine if it's a specific ram stick causing the problem.
6. If the video STILL is not showing up you will want to check the following.
Check the motherboard for blown capacitors; They will appear in the following forms; Clearly bubbled tops on the capacitors; Exploded tops or yellow crystalized liquid leaking from one or more of the capacitors (like battery acid). This causes a HUGE list of random issues that are symptomatic of power issues. In this case a (Failing Motherboard) Most common on intel and dell motherboards especially on older hardware/PC's.
The CPU is the hardest thing to detect if it's failing, and is rarely the cause, if common sense and care are taken when handling a CPU.
At this point if you still can't figure out what's causing the problem; Replacing the motherboard or monitor is recommended. Monitor if you do not have another monitor to test. Or motherboard if you've exhausted all other troubleshooting steps.
I realize it's a lengthy description, but your description of the problem wasn't exactly specific so I wanted to be thorough and cover all the basis that could cause this problem.
Hope this helps.