Here's random stuff I found that interests me...
Your NIC = Realtek... crap in my opinion. Could be the cause, but not particularly likely.
Your CPU = AMD... crap in my opinion. Could be related to the cause(very likely based on prior users and below info).
dmesg:
ACPI Error: [RAMB] Namespace lookup failure, AE_NOT_FOUND (20110527/psargs-392)
ACPI Exception: AE_NOT_FOUND, Could not execute arguments for [RAMW] (Region) (20110527/nsinit-380)
...
amdsbwd0: <AMD SB8xx Watchdog Timer> at iomem 0xfec000f0-0xfec000f3,0xfec000f4-0xfec000f7 on isa0
...
umass0: <vendor 0x1005 USB FLASH DRIVE, class 0/0, rev 2.00/1.00, addr 2> on usbus3
...
pid 1751 (vmware-checkvm), uid 0: exited on signal 10
That info from dmesg tells me the following:
Your ACPI support is not compatible with FreeNAS/FreeBSD. You could try rebooting the server and choose the menu option for acpi=disabled.
Your watchdog timer may be related to the problem. Watchdog timers reboot servers when they misbehave.
Your USB flash drive appears to be some crap no-name brand. You should be using a name brand ONLY. (this tend to corroborate with what I'm going to say in a minute.. keep reading)
For some reason some VMWare check tool exited. I don't think this is normal as your system should have booted up, recognized its not in a VM, then never loaded anything related. Could be wrong though.
Normally, if your system tries to boot from your zpool disks you'll get that stupid warning that says "this is a data disk". The fact that you woke up to that tells me that (1) the server rebooted itself and (2) the USB stick was not detected or (3) was not set as bootable by default in the BIOS or (4) is otherwise having problems.
So I'd get a new name brand USB stick and install FreeNAS on it. Import your config file, then bootup with ACPI disabled. See if that helps at all. I will warn you that some AMD boards have hardware that isn't compatible with FreeBSD/FreeNAS and the only solution is to get rid of that board. Unfortunately since I avoid AMD like the plague I can't really provide much help on how to prove its the motherboard other than using a different one.