I assume your have your homedirs on the NFS server. The SSH server probably tries to open the ssh private key in the user's .ssh dir and the OS fails to return any information. It also depends if your mount is done with the "hard" option or "soft" option. Usualy you only use "hard" if you are sure of the availability of the server, since a down server could hang your session:
Whether you hard- or soft-mount a volume depends partly on taste but also on the type of information you want to access from a volume. For example, if you mount your X programs by NFS, you certainly would not want your X session to go berserk just because someone brought the network to a grinding halt by firing up seven copies of Doom at the same time or by pulling the Ethernet plug for a moment. By hard-mounting the directory containing these programs, you make sure that your computer waits until it is able to re-establish contact with your NFS server. On the other hand, non-critical data such as NFS-mounted news partitions or FTP archives may also be soft-mounted, so if the remote machine is temporarily unreachable or down, it doesn't hang your session. If your network connection to the server is flaky or goes through a loaded router, you may either increase the initial timeout using the timeo option or hard-mount the volumes. NFS volumes are hard-mounted by default.
It's always safer to keep a local (non-root) login to avoid this kind of problem that would require you to physically move to the server to reboot it.