the vCenter Logs are located in %ALLUSERSPROFILE%\VMware\VMware VirtualCenter\Logs\. (ref http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1021804)
L ook at the latest to vpxd.log to know the reason for vpxd crash. We can see the crash details at the bottom of the file which created immediately the vCenter Service stopeed.