The feeling you get when you enter in your username and password into something important (Facebook, bank websites, e-mail, video games you've invested years of time into) only to find that your password doesn't work.
This often leads to a sinking feeling in your chest which quickly leads to increased heart rate and paranoia, and everything that is at stake flashes before your eyes like your money, private messages, or that +9 GTB Thorny Shield you spent years camping for.
Though typically, you haven't actually been "hacked", but really:
1. You have caps lock on, and your password is case-sensitive.
2. You typed the right password in right too fast, and somehow managed to fuck up three, four, or even five times. (See: failure)
3. After making sure you type each letter firmly and individually, you quickly realize that you:
A) Entered in the wrong username that looks similar (Johndoe696996 instead of Johndoe696969)
B) You changed your password, and you forgot you ever changed it.
If none of these are the case, then you have either been keylogged (Again, see: failure), or you have violated the Terms of Service and have been banned.
It turns out I had my caps lock on the whole time.