Disk suddenly read-only

[ubuntu at acrasis.net]

Why might a disk suddenly become read-only?  I have a 7.04 AMD64 server
with two disks, /dev/sd{a,b}.  The trouble is on sda which has
partitions for /var and /boot.  I can 'ls' the contents of these, but:

   # touch /var/foo
   touch: cannot touch `/var/foo': Read-only file system

   # touch /boot/foo
   touch: cannot touch `/boot/foo': Input/output error

/var/log/syslog ends with

   Jan 20 02:50:01 boron /USR/SBIN/CRON[8144]: (root) CMD (if [ -x
   /etc/munin/plugins/apt_all ]; then /etc/munin/plugins/apt_all update
   7200 12 >/dev/null; elif [ -x /etc/munin/plugins/apt ]; t

except that the final 't' is followed by 80 or so '^@' characters, as
rendered by emacs (the null character?).

What happened?  (How) can it be fixed?
-- 
KM