I recently ran in to an issue where in my XenServer test environment i seemed to lose all 4 of the NICs that was in my XenServer and the XenServer host could not be seen in XenCenter.
Looking at it closer when in XSCONSOLE and you looked at the management interface section you would get a message back saying "no interfaces present" and in the CLI if you were to run a xe pif-list you would get a message back saying "Host is still booting".
If you were to also run a ifconfig on the XenServer host would would even get back a list of all eth's, bond, xapi and xebr's.
I found out that running the xe pool-emergency-transition-to-master would resolve this issue. Once you have run this command wait around 30 seconds and then restart the host and it was now able to be seen within XenCenter.
Hope you found this information helpful.... You can also find it on my technical blog at http://www.kaztechsolutions.co.uk