Do you get a menu that appears before it loads Windows 7? Have you
successfully booted into Ubuntu? What changes did you make on the Windows 7
side that may have overwritten the boot loader?
The solution depends on whether grub still exists. If it no longer exists
but once did then you need to use the Live CD to re-write grub. If it still
exists then we need to figure out what it is doing and why you can't get
into (K)Ubuntu.
Another problem is whether this is going to be a chronic problem with
Windows 7 not playing well with sharing the drive with another OS. Does your
computer have a boot manager that allows you to choose which drive to boot
from by pressing the ESC key or similar? If so you could add a second drive
and put Ubuntu on it and writing the boot loader to the second drive then
when you boot you would need to use the ESC key to boot your second drive.
This is just a thought about what to do if Windows 7 continues to not play
nicely.