The problem is probably that your boot-partition does not have its boot flag set.
1.You need to download the Gparted ISO, burn it to a disc and boot from it.
2.Use Gparted to set boot flag of your boot-partition (probably the ext2 one).
Hope this helps!
[edit: of course, you can use any other tool besides Gparted to set the boot flag. I just like Gparted. If you run into trouble booting Gparted, selecting the Xvesa driver will most likely make things work.]