Launching Error!

When I launch G-Mod it will say “Preparing to Launch”, and than that will vanish, than an Error box pops up saying “SteamStartup() failed: SteamStartup(,0xf,0x0012EBC) failed with error 1: failed to take master pipe connection lock”
What should I do? I think maybe reinstalling it, but I don’t want to lose my addons.

Restart Steam.