If any new hardware has been recently added to the computer, remove that hardware to make sure it is not the cause of your issue. If after removing the new hardware your computer works it’s likely the computer is either not compatible with the new hardware or a system setting needs to be changed to work with the new hardware device.
Remove any disks, CD’s, DVD’s that are in the computer and if any USB devices, iPods, iPhone, drives, phones, are connected disconnect all of them as well. Reboot the computer and see if anything changes.
Remove everything from the back of the computer except the power cable. Turn on the computer and see if it beeps normally. If the computer has never beeped keep the monitor or display connected to see if any change occurs.
If you are receiving a sequence of beeps see the beep code page for a listing of different beep codes and their explanation or your motherboard or computer documentation. These beep codes are meant to help identify what computer component is failing or bad. If your beep code is not listed, continue troubleshooting.
Make sure all fans are running in the computer. If a fan has failed (especially the heat sink fan for the CPU) your computer could be overheating or detecting the fan failure causing the computer not to boot.
Verify that all the cables are properly connected at that there are no loose cables by firmly pressing in each cable. All disk drives should have a data cable and power cable connected to them. Your power supply should have at least one cable going to the motherboard. Many motherboards may also have additional cables connected to them to supply power to the fans.
If the above recommendations still have not resolved the irregular POST, disconnect the riser board if applicable and each of the expansion cards. If this resolves the issue or allows the computer to POST connect one card at a time until you determine what card is causing the issue.