you can try roolback to previous image via Shift+R during initial boot process of ESXi.
If its not possible you can try to catch host config via HostProfile feature (if you have license), reinstall host and try to attach and apply profile to newly installed host.
Before rolling back dont forget that VM hardware version 10 its not supported on 5.1 hosts
Is it possible to downgrade a host from ESXi 5.5 to version ESXi 5.0. I try to downgrade to ESXi 5.0 and getting successful but its showing
"VMware ESXI 5.5.0 ".
you may try steps in this thread (last post):