When it comes to cleaning up /boot, especially on an Ubuntu box, use the package manager to remove the kernels. You've wiped out the bootloader that boots your system, now you get to rebuild it by hand, or else reinstall. We'll go at it with the rebuild-by-hand method.
Boot your machine with an Ubuntu LiveCD. Once booted, you'll need to mount the boot partition from your hard drive.
Copy the grub directory from /boot/grub on the LiveCD to the mounted boot partition
Edit the menu.lst in the mounted boot partition, and modify the first entry to point to the kernel you have left on the disk.
This may get you going. Worst case, boot from the LiveCD, copy any data you need to a backup location and reinstall. Moral of the story: Use the package management included with your distribution to clean up your disk, and ALWAYS be careful as root.