If you can get to sites such as 'youtube' but the videos won't load it may be an issue with 'Flash', I know you said you reinstalled Flash but you must use the 'Flash Uninstaller' first or it will not correctly install (this goes for automatic Flash updates aswell - which kinda makes the auto updates useless). You can find the Flash Uninstaller here
Download it, then run it, then reinstall Flash
Some browsers use a different version of Flash, so install Flash for each browser.
(nb. If you're using IE8 you will need to download Flash for IE8 with another browser eg. Chrome (choose the 'different operating system or browser' and select IE on the Flash download page) otherwise IE8 crashes)
If you already did this then maybe you downloaded something in between it working and not working?
Can you get on to other websites?
Maybe try to roll back your system with 'system restore'.