I recently found  some VMs of one OVS node( from 30+ nodes)  went down and not able to start with this error :

Xend has probably crashed!  Invalid or missing HTTP status code.

There are many reasons behind this. And if you try to restart xend , it will not start.

The first place to look for is :

/var/log/xen/xend-debug.log

This log will say where exactly the issue is.

In my case my  / filesystem was running out of space because one log file consumed almost 8 GB . So I have to delete that file and now xend started successfully.