To 'hit the wall' is to suddenly reach a mental block. It could be as simple as running out of ideas or as complex as not knowing what to do in life. Hitting the wall usually happens suddenly and the effects can last for long periods of time.
While writing my newest novel I suddenly hit the wall and had no idea what to write anymore.
The point during the work week in which you have lost all motivation to perform you job duties. Productivity decreases significantly after hitting the Friday wall, while the amount of time spend surfing the Internet and generally not doing anything productive increases.