My computer came with Windows Vista, and I later dual-boot installed Ubuntu. Recently, I upgraded Windows Vista to Windows 7, and since then, the computer no longer asks which operating system I want to boot from. So this means I no longer know how to boot Ubuntu at all. I'd prefer not to have to reinstall Ubuntu unless absolutely necessary.
Any idea what I can do to fix this?
Actually Windows clears the Bootloader and does not care about other OS installed.
So you dont have option now , If you install Ubuntu now , (Ubuntu being Ubuntu ) it will take care of Windows and will give you option to choose.
Read here , it talks about how to fix this.
https://help.ubuntu.com/community/WindowsDualBoot
A Windows OS should be installed first, because its bootloader is very particular and the installer tends to overwrite the entire hard drive, wiping out any data stored on it. If Windows isn't already installed, install it first. If you are able to partition the drive prior to installing Windows, leave space for Ubuntu during the initial partitioning process. Then you won't have to resize your NTFS partition to make room for Ubuntu later, saving a bit of time.
Jadhaik, Go to : https://help.ubuntu.com/community/Boot-Repair I once had same problem. Used this Boot-repair software and it fixed it for me.
In my case, after I installed Ubuntu, my Windows 7 was not booting. So I inserted Windows 7 installation disk and fixed Windows Boot. Hence, I could not boot Ubuntu, it was always booting Windows 7.
To fix it, first, I booted Ubuntu from Pen-drive. Then installed "Boot-repair" in live-usb. Using Boot-Repair, I applied "Recommended Repair". And Done(Saved me from re-installing Ubuntu). It is available as "Boot-Repair-Disk" too. Check link on Boot-Repair page.
