vCenter Appliance Stops working

hi guys

all of a sudden my  vCenter Appliance 5.0.0-455964 stops working I mean it works it stops and sometimes I have to reboot it or I just need to restart the vcenter service but most of the time I have to reboot it.

so my question is where I should look for logs I already checked vmware-vpxd.log but it does not mention anything about why it stops working.

any other log to check guys?

thanks a lot