I had Ubuntu 11.10 installed, I installed Windows 7 on another partition but now on booting I cant see dual boot options and Windows 7 is loaded.
How can I run Ubuntu now?
Your grub2 installation was removed from the MBR by the Windows installation.
Please follow the instructions in the Recovering Ubuntu After Installing Windows wiki page, the graphical method should restore your grub2 install that was over written by the Windows installation.
The recovery method should detect both your new Windows and the Ubuntu install.
Is seems you have to reinstall grub as it was wiped after installing Windows 7. If I'm correct you will be looking for reinstalling grub 2. There seems to be some options for restoring grub 2 given on the ubuntu community help site: https://help.ubuntu.com/community/Grub2#Reinstalling_GRUB2