I fired up the computer today and as I selected the last Sonar project I was working on, the computer shut down. When it came back I got the usual black screen telling me the computer was not shut down properly. I chose Start Windows Normally. It tried to go through its process but then shut down again. Then it was just stuck in a loop trying to restart for a second or two then shutting off. Has anyone experienced anything like this before? I'm wondering if it's the HD or maybe the power supply.

Could be the power supply or even a loose power connector to the motherboard.  But it could be the motherboard, too.  It probably is not the HD based on what you described.
 
Are you getting any beeps from the power on self test?

Reseat the main and secondary power connections to the motherboard.
When a machine is "stuck in a loop" (where it starts to spin up then stops - repeatedly), it's typically the power-supply or motherboard.

I just swapped out the PS with a good one I had laying around. Same problem.
 
Dave, what do you mean by "self test"?

If you've swapped out the PS, it's almost surely the motherboard.

Jim Roseberry
If you've swapped out the PS, it's almost surely the motherboard.

Dang.

I had a P8Z77LE motherboard and it died almost like yours, and of course out of warranty. Motherboards go through a "POST" (Power On Self Test) when powered on.  On this model there are beeps if there is a problem.
Page 42 in the manual details the beep code. 
One short beep, VGA detected but NO Keyboard Detected.
One continuous (long) beep followed by two short beeps, No memory detected
One continuous (long) beep followed by three short beeps, No VGA detected
One continuous (long) beep followed by four short beeps, Hardware Component Failure. 
 
If it turns out to be a motherboard, considering the age it is probably time for a new system.
 
 

fireberd
I had a P8Z77LE motherboard and it died almost like yours, and of course out of warranty. Motherboards go through a "POST" (Power On Self Test) when powered on.  On this model there are beeps if there is a problem.
Page 42 in the manual details the beep code. 
One short beep, VGA detected but NO Keyboard Detected.
One continuous (long) beep followed by two short beeps, No memory detected
One continuous (long) beep followed by three short beeps, No VGA detected
One continuous (long) beep followed by four short beeps, Hardware Component Failure. 
 
If it turns out to be a motherboard, considering the age it is probably time for a new system.
 
 

Yeah, no beeps. You can hear it spinning the drives and then it shuts down. This is the first board that didn't make it to 6 years. Ugh, I was not ready for a new system yet.

I was able to find another P8Z77-V LK on ebay and after installation and powerup, I'm getting the same startup/shutdown cycle again. However, it only does it about 5-6 times then remains powered up. The problem I'm having now is that I'm getting nothing on my display. I switched my monitor between the graphics card and the onboard video and still nothing. Also, I'm not getting any beeps.
 
Update: On a whim I removed one of the DIMMs and the system started up. So I reinstalled my original MOBO with only that one DIMM and it's up and running again. Strange that I never got any memory errors.

Always test your RAM one stick at a time.