There is no way to simply undo whatever erased the associations. The easiest way to is to perform a hard reset. This will restore any lost associations. Backup your info first.
Once your done re-install your programs and info. If you restore the registry from the old backup you may lose everything again.
The hard way would be to go into the registry and manually re-enter the missing associations. But to do that you would need to know exactly how they originally appeared in the registry. I've seen it and it's not a 1,2,3 your done kinda thing. So good luck with the reset.
