Hi, I've got 3 nuggets of help to offer you today.
The first nugget is to try running YouTube in compatibility view (click that broken piece of paper next to refresh). If this doesn't work please try the next two nuggets of help.
The second nugget is to install Google Chrome, you can use Google Chrome until Adobe start supporting Flash for IE8 x64 or any other browser for that matter.
The third nugget is if YouTube is the only flash site you want access to, as long as you have Google Chrome installed, you can run YouTube in HTML5 not flash thus removing the flash problem element entirely. You can opt into YouTube HTML5 by going to YouTube - Broadcast Yourself.
You could also have a look at running IE x86, there is a thread about how to do that at Flash Player for 64bit IE
As for YouTube working previously, I have no idea
I hope this is of some use.