I had this happen to me 4 times I think on the sam customers vCenter sometimes when we rebooted the Windows server for Microsoft patching the vCenter webpage would load with just headings no configuration or content. Which I must admit the first did make you panic a bit.
Searching the web and talking to other peers only answer people could recommend was to rebuilt vCenter an option, but, not the best solution.
So I logged a ticket with VMware the issue appeared to be with the vCenter Postgres DB having corruption. The engineer to took a backup of DB and VMware investigated this and came back to me and resolved the issue.
To resolve this issue I would recommend you have a backup of your vCenter DB this can be done by following this VMware KB2091961 and logging a ticket with VMware