No display on computer during boot up

Try a deep power cycle, which will hopefully resolve this problem for you (it has for some of my clients who've encountered it on rare occasions):

1. Shut down the computer and power it off
2. Disconnect the power cable
3. Attempt to power on the computer using the power switch (it won't power on)
4. Re-connect the power cable
5. Attempt to power on the computer using the power switch (it should power on)

If this doesn't resolve the issue, then you probably have a malfunctioning power supply (testing equipment doesn't always tell us everything) or motherboard (hopefully it's only the power supply as this is a lot less expensive to replace).

Also, most motherboards do provide a display immediately after powering up, but some do present a bit of a delay first (especially certain models of server motherboards which can delay for up to 5 minutes before activating the display -- these motherboards are particularly annoying when you're dealing with a server problem, and even stressful, even though the vendors continue to insist on doing this by design).