I assume /dev/sda7 is the root partition of Ubuntu. Try booting up the Ubuntu live CD, open a terminal (applications > accesories > terminal) and run "sudo fsck /dev/sda7" (without the quotes). If this works ok you should be able to reboot into Ubuntu from the hard disk.
Did you just install Ubuntu and boot it for the first time and get this error? Or has Ubuntu been running ok for a while and this just happened all of the sudden. Did you repartition your hard dive after the install?
Could you post the output from terminal of:
sudo fdisk -l (that is a lower case letter "L")
I'm wondering why your root partition is /dev/sda7. Do you have other operating systems or Windows partitions on the machine?
EDIT: also, see this thread: