Don't worry, Nobody will go hard on you, We're a helpful, Friendly community. (For the most part
)
421 is the GameID of Episode 2, So try verifying HL2:Ep2 Cache by right-clicking on it, Properties, Then verify integrity of cache, Or better yet, Try re-installing it.
I've had a similar issue before and reinstalling HL2:Episode 2 fixed it.
If that doesn't do the trick, Someone will most likely be able to help you better.