Success means that the UML host has started your guest process. Your Linode is crashing during boot and once the host sees this, it reverts your Linode Platform Manager status to 'Powered Off'. Ssh into your Linode's console (ssh
linodeusername@hostxx.linode.com, rather than ssh
me@mylinode.tld) and look at the log to see where it's failing.
Most likely, you have one of the 'latest' kernels selected in your default profile and the kernel version has changed since your last successful boot. The biggest recent change is that devfs has been removed from the kernel, so if your distro uses it, you need to make changes (e.g. Gentoo /dev/ubd/0 needs to be /dev/ubda).