It could be if you have a virus scanner or something like that the virus scaner has blocked roblox. Try re-downloading roblox if not its your virus scanner or...something else is blocking it.
Hey.
Try clearing your browser cookies. If that doesn't work, you can install a fresh new copy of ROBLOX at roblox.com/install.
However, you should be fine after clearing your browser cookies.
Thanks,
Jason Kinetec