I Need HELP Gmod and other source games will not open!

I was playing gmod it worked perfectly fine then it crashed and now gmod and any other source game will not open I keep getting a message that says:

“SteamStartup() failed: SteamStartup(0xf,0x0015E3B4) failed with error 1:
failed to take master pipe connection lock”

What does that even mean?

Please I am Very DESPERATE! I Miss Gmod and Team Fortress 2!

Restart your computer.