In addition to the common OpenStack CLI commands like openstack server list here are some typical commands to troubleshoot an instance from the command line:
|Command||What it does|
|openstack server show UID||shows you the state of the instance and any OpenStack errors|
|openstack console log show UID||shows you the running console log of the instance (same as doing "dmesg" from the command line of the instance itself)|
|openstack console url show UID||yields a URL you can paste into your browser to get a login console for your instance|
Usually a combination of these things will let you see what's going on and what to do to fix. You should also be able to do all of these things from Horizon, as well.