Hi and welcome to LQ.
You should be able to use a repair option with the ubuntu cd.
do you get a boot menu like
1. boot from HD
2. boot from Ubuntu cd
3. repair system (system rescue)
The rescue option with mandriva allows re-installing grub which,
during the process searches for the file systems, confirms if this is what you want.
see how you go.
BTW, LQ has a fantastic search function where you can check for similar questions.