tar is in /usr on a lot of systems, so it's probably gone too. The boot process probably needs a few tools from /usr as soon as it can be sure that it's available.
IMO, your best bet is probably to boot from a rescue medium (eg Linux Live-CD), use that to backup any leftover data, reinstall and then restore.
Lessons learned:
- Always keep your backup current
- Always check your commands before you hit enter
- Always test on a expendable system first