Hard to say without much description of the problem, but it could be a problem with Grub (the bootloader).

Try burning this to a CD: http://www.supergrubdisk.org/super-grub2-disk/

Direct download link: http://download.berlios.de/supergrub/super_grub2_disk_hybrid_2.00s1-beta1.iso

Burn it to a CD and boot from it like you did when originally installing Ubuntu. Then ask it to detect operating systems. If it shows Ubuntu, select that and see if it boots.

Report back if it works.