over the 6+ yrs there have been issues with Mac Pro when left alone for 1-2 weeks.
There is no clear cause or cure. Having it on UPS is best. Unplugged for that time has been blamed but hey, mine is unplugged for weeks and months at times during these 6.75 yrs.
The methods for resetting SMC have changed. And it partially depends on WHICH Mac Pro model you even have.
Ideally everyone has a bootable copy (clone) of their system that they keep on shelf - off line and safe - at all and any given time.
Free space alone is not an indication of bad sectors, bad directory, electrical "event" or not.
Having a 900W UPS / 1500VA is as good as weakest link, not everyone puts modem and router and EJ45 through surge filter, the CyberPower SMART 1500VA works best I've had after using APC for over decade.
Check is system profile showing all the RAM or not? parity issue? other?
Are you on Snow Leopard? Mountain? use Disk Utility Repair Disk from another boot device.
Remove ALL cables and power and hit power on for 10 seconds and then connect only the kb/mouse and dispaly.
If your computer won't turn on, try each of these steps:
- Make sure that the power cord is plugged into a working wall outlet, and that it's properly connected to the power adapter. (To check if the wall socket is working, plug in a lamp or other electrical device.)
- Make sure that the power adapter is properly connected to the power port on the side or back of the computer.
- Disconnect all accessories that are plugged into the computer, such as a printer, hub, or other mobile device.
- Reset the computer's PMU, SMC, or PRAM. Refer to the instructions for your model:
reset SMC For "Mac wont' turn on or boot" and related
Note: It is possible that the SMC could encounter an issue that may cause unusual system behavior typically related to the symptoms described below. In some cases, resetting the SMC may be the only correct method to resolve the issue,
https://support.apple.com/kb/TS1367
General purpose Mac troubleshooting guide:
Isolating issues in Mac OS X
Creating a temporary user to isolate user-specific problems:
Isolating an issue by using another user account
Identifying resource hogs and other tips:
Using Activity Monitor to read System Memory and determine how much RAM is being used
Starting the computer in "safe mode":
Mac OS X: What is Safe Boot, Safe Mode?
To identify potential hardware problems:
Apple Hardware Test
General Mac maintenance:
Tips to keep your Mac in top form