Do you have IPMI (or equivalent) support on your server? If so, connect via that interface and troubleshoot the issue.
If you don't have IPMI, you'll probably have to connect a monitor to your server in order to troubleshoot the issue.
If you want answers, please post detailed server information, per the rules in red at the top of the page. What version of FreeNAS did you have before the update, what version did you update to?