Sorry to hear about your troubles, we've all been there.
Did you have N4 or N3 installed on your machine before installing N6? Because when I installed N5 over N3 it caused me major headaches in N5. When I wiped my drive clean and installed N5 first, and then N3, all was OK. However maybe if I had trashed the preferences folder for N5 as is often discussed here, maybe that would've precluded needing a fresh install - but I wasn't aware of that option at the time.
Lastly, it is my understanding that your N6 license covers all previous versions, so if you can ask somebody here for an ISO installer of N5, you should be good to go. It seems that when I check the Steinberg downloads page, that only the updates for N5 are available, but not the original program.
Studio Ray NY
Nuendo 6.04 + NEK, and 5.5.5 and 126.96.36.1993, Wavelab 4, Win 7 64 bit, Q6700, 6Gb Ram, 2 x UAD-1, RME Multiface, Motu 2408, Motu MTPAV