Did someone set up the RAID 0 for you? If so, I'd refer this question back to him.
If you set it up yourself, then you probably need to go into the RAID BIOS and see if there's any way to re-enable the failed drive. If not, you're data is probably gone and you'll have to get a new drive and start over again from scratch.
RAID 0 performs better but has no redundancy and is less reliable than a single drive. A drive failure means you loose all your data.
if just one of the two hd's fail in a RAID 0 array, you lose ALL your data..
to fix this, you should find another hard drive of similar specs if not the same hd to recreate a raid array..
i have 2x 250gb in raid 0 with a 1tb hd for backup/storage.. since i know that if i lose 1 hd in the raid, ill lose all my data, i backup anything i need from the raid onto my slave..
not sure if you have to delete your current broken raid array before you recreate another one.. but you would have to "create" the raid array before you can use raid again..
maybe you might want to look into more backup solutions or consider not using raid if you dont care.. just an opinion
You normally press a key like F2 or DEL during the boot sequence. But I have to say that if you're unfamiliar with using the BIOS then you're probably going to need someone to help you through this...