when I start opposing fronts it says it can't validate the media

is the disk in?

i would work through the troubleshoots, support etc.

try restarting steam
try signing out of steam then logging back in
try deleting game then reinstalling
or
contact steam and let them know the games gone tits up and it won't work.

Oh it used to do that shit to me all the time, and I owned the disc version.
For me it was a random thing, sometimes it'd work sometimes not. :/