Switching to runlevel 1 kills all processes (except the top-level
init/upstart command itself), including the SSH daemon.
Run Level 1 is known as ‘single user' mode. A more apt description would be ‘rescue', or ‘trouble-shooting' mode. In run level 1, no daemons (services) are started. Hopefully single user mode will allow you to fix whatever made the transition to rescue mode necessary.
The easiest way to get
sshd running is to switch to a runlevel that starts it by default. In Ubuntu, that's any of 2, 3, 4 and 5.
If you can't access the single-user shell to enter the
telinit command, eg. because you were connected remotely, or it's hidden by the splash screen, then you're out of luck. The only option left is to reboot.