You run the game back up, enter your password, and lo and behold- your password is incorrect.
AKA Error 313500.
But this can't be true! I was just logged in! And I'm pretty damn sure my account isn't hacked.. (I hope?)
Fear not, young Timmy! All you have to do is follow these simple steps:
1. Exit Diablo (obviously.)
2. Open your Diablo III directory (usually ..\Program Files\Diablo III)
3. Proceed to the Data_D3 folder
4. Proceed further into the PC folder (you don't want to get confused with your PS3 version!)
5. Proceed even deeper into the realms of the unknown, AKA your MPQs folder
6. Locate the folder called "Cache" and delete it.
7. Restart your computer. I have no idea why, but this step is crucial.
8. Right-click your Diablo III icon, and click "Run As Administrator"
9. Congratulations! Your Password is valid once again!