"that worked; nothing"
Um, okay, so which was it? It worked? Or nothing [happened]?
While they may be other possibilites to fix, I suspect you are on your
way to booting from the install CD and doing a repair (i.e., inplace
installation).
Afterward be sure to reinstall any service packs and updates. If you
installed SP-2 but your install CD is SP-1, you need to slipstream SP-2
onto a bootable CD (but I'll get into that later since it may not apply
to you, and fact is that I have repaired using SP-1 and then simply
reapplied SP-2 right after).