503 Service Unavailable error is usually caused by full-service did not start. From the beginning of 6.5 version, vc web are logged.
If vcsa is installed, you can log in to this exsi where vc, where vc open into its virtual machine Shell (alt + f1 by at vcenter Interface) interface to perform the following command:
# Control-Service--all
enlarge many services to
VMware-invsvc
VMware-MBCS
VMware-netdumper
VMware-perfcharts
VMware-rbd-Watchdog
VMware-SPS
VMware-the VDCs
VMware-vpostgres
VMware-VPX-Workflow
VMware-the vpxd
VMware-VSAN-Health
VMware-vsm
VMware-VWS
vsphere- client
if there is some services did not start, such as
VMware-MBCS
VMware-netdumper
VMware-rbd-Watchdog
Run the command confirmation, first of all execute the command:
# -status VMware service-Control-netdumper
You can see the service in the Stopped state, the service is to use VMware Message Bus Configuration Service;
Then execute the command:
# -status VMware Service-Control-netdumper
See these three services are at a standstill, then the next attempt to manually start them with the following command:
#-Service Control -start-rbd-Watchdog VMware VMware VMware-MBCS-netdumper
see all the services are started, and finally again when the vSphere Web Client interface to refresh, you can successfully log in to your VCSA the (of course, pay attention to service has started dependencies)
If you put vc installed in the windows above
You can go to the bin directory to operate 
to stop the service has already started, you can enter up to vc interface
Another way is to re-build a vcsa, the exsi host and resources are migrated in the past, but would reconfigure some trouble.