I just installed the latest version of Ubuntu next to my Windows 7 system. After I restarted my computer, it started Windows 7. I looked under Computer if I can start Ubuntu under Windows 7, but it does not show me the partition where I installed Ubuntu with 3 partitions. I restarted again, went to boot menu, but wasn't able to start Ubuntu. What can I do?
Since you mentioned you have used three partition I will assume you want a dual boot not an wubi install.
Boot using a live usb/cd of ubuntu.
boot-repair to fix the problem.
After booting with live usb/cd ,Run following command in terminal,
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update sudo apt-get install -y boot-repair && boot-repair
At the end it will give a link , note it down before rebooting. If the above doesn't work just post that url here.
More information can be found here- > https://help.ubuntu.com/community/Boot-Repair
Assuming you installed your Ubuntu system alongside Windows (by booting from a CD/DVD or USB flash drive and running the installer), and not inside Windows (with the Wubi installer), this problem can almost always be solved easily by reinstalling GRUB2 to the Master Boot Record, using any of these methods.
If you installed using the WUBI installer then when you restart your machine it should prompt you from the boot windows boot menu. you can choose to boot either your windows install or the Ubuntu. Were you not given this option when you restart he machine?