I have seen ssh stop working a few times on a rb450g running 5.0 rc1.
This gets it running again:
/ ip service
disable ssh
enable ssh
The time before it fails, it prompts for passphrase, and even with the correct passphrase cut 'n pasted in, it fails. After that, no response on port 22 until I restart. Then ssh login works fine.
yes, that disable/enable stops (if working) and starts (if not working) ssh server. If after that it starts to work please make supout.rif file and send it to us, so we can fix the problem.
I have recently started having this issue on an x86 machine running 4.7. I upgraded it to 4.11 and ran into the same issue about 4 days later. Rebooting has fixed it, I will try disable/enable the ssh service if it happens again.