How to find out the reasons why the server went into reboot?

Question

Windows Server

There is a VMware ESXi 5.5 U3 hypervisor with several VMs with OC Windows 2008 R2 and Windows 2012.
One of the VMs with OC Windows 2008 R2 went into reboot for the second time in a week, help me figure out where to look?
This entry is in the log. Maybe there are more magazines where you can find out in more detail what the reboot is connected with.
The rest of the VMs on this host did not reboot.

Log Name:      System
Source:        USER32
Date:          4/24/2017 4:14:01 PM
Event ID:      1076
Task Category: None
Level:         Warning
Keywords:      Classic
User:          admin
Computer:      SRV1
Description:
The reason supplied by user admin for the last unexpected shutdown of this computer is: Other (Unplanned)
 Reason Code: 0xa000000
 Problem ID: 
 Bugcheck String: 
 Comment:

Answer 1

There was an emergency reboot. Most likely, no traces in Windows at this stage can be found.
The reasons can be either in hardware or in software.
For hardware: this may be due to overheating of the processor; due to the fact that any PCI adapter physically departs - poor contact; failure / overheating of the power supply; just the old motherboard started to fail.
Turn on any monitoring with saving to a file, even if it is built into Windows and wait for it to happen again. Also look at BIOS - normal server manufacturers have their own log in BIOS and hardware errors are written there.
By software - you know better what you have there. Update the software, turn on the logs, analyze after repeating. If all else fails, try dragging some of the software to another server, if possible.
Of course, all this makes sense if the mistake is repeated regularly. If there are no repetitions, then score.
I had one server under ESXi, which, after updating ESXi, began periodically crashing into BSOD every 3-4 months. Because the crashes were very infrequent, then it was not possible to understand the reason - the hardware is working, there is nothing in the ESXi logs. Tried several options for a couple of years ... now this server works fine under Windows.

Answer 2

logs from the hypervisor

/var/log # cat vmkernel.log | grep 'Failed to open file'

2017-04-25T05:50:49.742Z cpu3:37367)FSS: 5914: Failed to open file 'SRV-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219450 [vmx-vthread-5:SRV]): Busy

2017-04-25T05:50:49.757Z cpu3:37367)FSS: 5914: Failed to open file 'SRV_1-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219451 [vmx-vthread-6:SRV]): Busy

2017-04-25T05:50:49.761Z cpu3:37367)FSS: 5914: Failed to open file 'SRV_2-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219452 [vmx-vthread-7:SRV]): Busy

2017-04-25T05:50:49.764Z cpu3:37367)FSS: 5914: Failed to open file 'SRV_3-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219453 [vmx-vthread-8:SRV]): Busy

2017-04-25T05:51:16.333Z cpu9:3010997)FSS: 5914: Failed to open file 'SRV-flat.vmdk'; Requested flags 0x40001, world: 3010997 [vpxa-worker] (Existing flags 0x4008, world: 16219450 [vmx-vthread-5:SRV]): Busy

2017-04-25T05:51:16.337Z cpu9:3010997)FSS: 5914: Failed to open file 'SRV_1-flat.vmdk'; Requested flags 0x40001, world: 3010997 [vpxa-worker] (Existing flags 0x4008, world: 16219451 [vmx-vthread-6:SRV]): Busy

2017-04-25T05:51:16.340Z cpu9:3010997)FSS: 5914: Failed to open file 'SRV_2-flat.vmdk'; Requested flags 0x40001, world: 3010997 [vpxa-worker] (Existing flags 0x4008, world: 16219452 [vmx-vthread-7:SRV]): Busy

2017-04-25T05:51:16.344Z cpu9:3010997)FSS: 5914: Failed to open file 'SRV_3-flat.vmdk'; Requested flags 0x40001, world: 3010997 [vpxa-worker] (Existing flags 0x4008, world: 16219453 [vmx-vthread-8:SRV]): Busy

2017-04-25T06:19:25.451Z cpu0:34532)FSS: 5914: Failed to open file 'MSR11WST4130788-flat.vmdk'; Requested flags 0x40001, world: 34532 [vpxa-worker] (Existing flags 0x4008, world: 16908892 [vmx-vthread-5:MSR11WST4130788]): Busy

2017-04-25T06:19:46.516Z cpu7:37367)FSS: 5914: Failed to open file 'SRV-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219450 [vmx-vthread-5:SRV]): Busy

2017-04-25T06:19:46.519Z cpu7:37367)FSS: 5914: Failed to open file 'SRV_1-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219451 [vmx-vthread-6:SRV]): Busy

2017-04-25T06:19:46.521Z cpu7:37367)FSS: 5914: Failed to open file 'SRV_2-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219452 [vmx-vthread-7:SRV]): Busy

2017-04-25T06:19:46.524Z cpu7:37367)FSS: 5914: Failed to open file 'SRV_3-flat.vmdk'; Requested flags 0x40001, world: 37367 [vpxa-worker] (Existing flags 0x4008, world: 16219453 [vmx-vthread-8:SRV]): Busy