Recently I had my XenServer die on me, which I'm assuming was the result of starting too many VMs simultaneously. The symptoms were that it was unreachable from the XenServer Client application, however I could ssh into it, and some of the VMs were running.
This appeared to be a problem with xsconsole. I tried to restart it with "service xapi stop" however this never returned from the prompt (it hung).
To fix this problem I found this post
which was basically:
killall -9 xapi
This worked in stopping the service, and I was able to start it with "service xapi start."
I did notice some glitches still, like when I attempt to start one of my VMs I got an error about some bubbles not being available.
Thus, since I now had access to the XenServer via the Client application, I cleanly shut down all my VMs and entered/exited maintenance mode, and rebooted the server. This seemed to fix my problem, as all of my VMs came up fine after that.