"Failed to sign into Playstation Network" error?

Every time I attempt to use any online features a message pops up titled, "FAILED TO SIGN INTO PLAYSTATION NETWORK" that says, "You failed to sign into PlayStation Network. All online modes and features will be disabled until you sign-in.
I attempted to do so in game, but the sign-in button refused to work. I checked to see if I was actually online and my friend verified I was.
Apparently this issue has occurred to other people as well. However, I've failed to find a solution for it. I've only heard of one, which was to sign-out through the PSN menu and then sign back in within the game.This didn't work; the game did sign me into PSN, but it still kept telling me I wasn't actually signed in.
Nothing I tried so far has worked. How am I supposed to fix this?