I cant believe that apt is missing from ur ubuntu.may be becoz of some shell env variables are not set correctly
for eg: PATH.if apt is removed(less chance) then aptitude & other gtk dep tools like synaptic etc too are removed.this is either a kernel error or missing PATH variable
.also make this clear which OS's are u have on ur system,what did u tried and do u still have grub menu for ubuntu there-then try below method.
So just try booting ur ubuntu to runlevel 1 and check for errors.like press "e" on grub menu for ubuntu and then press again on line starting with "kernel "
then remove the last lines " ro quiet splash" and add "rw init=/bin/bash" to the end of the line.press enter;press "b" to boot to get a root prompt.then their try "/usr/bin/apt-get install -f" and note for any errors.
below is chroot method:
dont try this now
use ur ubuntu livecd configured with internet=better.mount ur ubuntu- /,/boot etc partitions correctly.chroot into ur ubuntu partn.try dpkg --configure -a and note the errors.