On my 904HD with win XP I installed Ubuntu.
One Day Grub got messed up and not having a DVD Drive, not able to reboot System CD!?
What can I do?

One, you can get an external CD/DVD drive and use that with your CD (if you have a USB hard drive adapter thingy, it might be possible to use that with an internal CD/DVD drive).

Two, you can create a bootable USB stick (I believe UNetbootin is a program you can use for this), boot from that, and use that system to fix grub.

Three, you can pull out the drive from the netbook and put it into another PC as a secondary drive, and use that PC to fix grub. (This assumes it's a normal HDD - I'm not familiar enough with your model to say for sure).

I would recommend #2 as the easiest approach, unless you happen to have everything you need for #1 but not for #2.

Oh, and Four, you can take the computer and go bother a friend who's good with this kind of thing. ;)