stupid me i need sos

i was trying to make a backup of /var/log/messages and i tried to cp /dev/null to /var/log/messages but lo and behold i did mv /dev/null /var/log/messages, now i can log in i get error sayins invalid filesystem i had to boot with emergency disk, how do i restore /dev/null and what is the problem that i caused -- SuSe 7.0 Linux 2.4.2 i686 Wed Apr 25 19:05:00 EDT 2001