Repair grub

If you dont see boot menu at startup and cant access to your ubuntu, it means you have grub problem..... Insert your Live cd or usb and boot it from there.... Then open terminal and type
sudo grub-install --root-directory=/media/YOURFILESYSTEMLABEL /dev/sda
FILESYSTEMLABEL is the name of disk partition you have installe Ubuntu.. Actually it is named UBUNTU.. fore sureness go to System>>Administiration>>Disk Utility and click on xxx GB Hard Disk (xxx is size of your hard disk)... Check out where your ubuntu is placed... /dev/sda is your device, to be sure look at the top write and you will see Serial number, under that World Wide Name, and under that Device : /dev/sda or may be hd0..if it is hd0 then write hd0..
if sudo grub-install --root-directory=/media/YOURFILESYSTEMLABEL /dev/sda command returned with error then type
sudo grub-install --root-directory=/media/YOURFILESYSTEMLABEL/boot /dev/sda