When you do not have a CDROM to boot then you can use alternatives, such as a floppy disk, an USB pendrive or boot from the network.
In your case booting from USB is probably the easiest solution.
Once you are booted from the alternative media you can use repair your system. I never did this with Ubuntu, but according to https://help.ubuntu.com/community/Boot-Repair is is as easy as:
- Booting from the pen drive,
- Opening a shell and using installting bootrepair with this command:
sudo add-apt-repository ppa:yannubuntu/boot-repair
- Once it is installed ed
sudo apt-get update && sudo apt-get install -y boot-repair &&
(boot-repair &)
Regardless of these actions or how you fix it, something broke it in the first place. Try to remember what happened prior to the bootloader failing. Did it run out of battery? Did you shut it down hard without unmounting volumes? ...