SteamStartup() failed: Missing Interface

What does this mean and why is it showing up?
The mod itself just won’t launch anymore.
What the heck is going on? :[

So…after I restarted steam…
Every time I launch it says "Failure to start game. (missing executable)
I followed the steps to fix that but it’s not working.
So…
I can’t finish this game now.

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.