Solution
Ruling out a 3VR issue:
Is the power light on?
E-Series: steady blue power indicator light next to the power button
S-Series: steady blue power indicator light next to the power button
S-Series 2: steady white power indicator light above the power symbol on the front of the unit, below the 3VR Logo
P-Series: steady pinkish-white light from the 3VR logo on the front of the box
M-Series: steady green light on the front
Are the system's cooling fans on? (Not to be confused with the fan directly on the S-Series power brick.)
Is the hard drive activity light on (steady or flashing) or any internal LEDs visible from the back?
E-Series: green LED on the front, flashes during activity and steady during chkdsk
S-Series: blue LED on the front, flashes to amber
S-Series 2: green LED on the front flashes during activity, flashes at steady interval if chkdsk or rebuild
P-Series: green LEDs on each HD bay behind the front panel
M-Series: amber LED on the front
Disconnect the system from the wall completely and let sit for 2 to 3 minutes, then try to power on again.
Does the system emit a beep or series of beeps when the hitting the power button? This indicates a problem with the RAM or a bad RAM port.
Note in case: Have there been any recent power issues (failures/surges)?
For units shipped prior to Q1 2010: lights indicate power, but not activity so all 4 will be on and solid during normal operation
For units shipped after Q1 2010: one light indicates data access which will blink and the other will be a power indicator which will always be on when the system is powered up
If the system is obviously receiving power but boots to a blank, black screen, see solution