Can't enter Ubuntu 12.04 after installing Windows on a separate partition

Solution 1:

You must repair the grub (windows has corrupted it). You can easy do this with de boot-repair disc. Boot with this disc, and let repair the system automatically.

https://help.ubuntu.com/community/Boot-Repair . Good luck

Solution 2:

I take it you installed Windows after Ubuntu. Windows doesn't know about (or care about) other operating systems, unlike Linux. So, it automatically installs its bootloader in the MBR.

I think the easiest way is to get the Boot Repair Disk, which can fix this automatically. It's saved me many times in the past. It can be done with the original Ubuntu install disk, too. The basic task is to install grub back into the MBR, and tell it to look at the Ubuntu partition for its menu. It's a little more involved doing it this way, though.