Kernel Linux 3.5.0-20-generic freezes at login stage of booting

Kernel Linux 3.5.0-20-generic freezes at the login stage. This only started within the last 12hrs. I don't know if there's been a kernel update recently as Grub automatically boots to the latest kernel unless specifically instructed otherwise. I can boot using Kernel Linux 3.5.0-19-generic or earlier. Any ideas as to how I solve this. I'm a non professional/technical Ubuntu user of 6 years with very basic terminal knowledge.

Regards