Looks like there's some problem with the ext filesystem these days... because this is the 4th time i get this question in the last 2 weeks. (from friends and now from here).
My guess is that you have the filesystem corrupted or broken in any way, but it's easy to fix, don't worry ;-)
Everything can be done from the Ubuntu live CD. Boot from there, and check which one is the partition where you have ubuntu installed on (sda1, sda5...). You can do it easily using Gparted. Even from there, you can right click on the partition and select "verify".
If you want to do it from the command line, just type:
fsck /dev/sda1 (being sda1 the partition where you have ubuntu installed).
And it will fix the ext4 filesystem problems. If you're getting too much questions while using it, add the "-a" switch to it:
fsck -a /dev/sda1
I have to tell you that use this with precaution. If the data is sensitive there, first make a backup if you can from the livecd to someplace else.
After the filesystem is fixed, you should be able to boot normally in ubuntu again.
Hope this helps you with that.