I accidentally made my file system read only. How can I change it back?
It sounds like you've accidentally changed the permissions of /etc/fstab. You should be able to use sudo because /var/run/sudo is mounted on the /var/run filesystem (or /run for Oneiric and up).
First, make the root partition writable again:
sudo mount -o remount,rw /
Next, restore the permissions:
sudo chown root: /etc/fstab
After that, future boots should not make everything read-only.