I have a 2001 jaguar s type 4.0 it was running fine but it had shut off we I was leaving for work I look under the hood and the hose closest to the throttle housing was off I put it back on and the car won't start anymore it say failsafe mode on the dash help

 

You have posted on the XJ (X350) board, may be a Mod will move your post to the S type board, where more people with the 4.0ltr V8 AJ engines.

These guys have the 4.2. on here

Any more details of the hose and what it is, cooling water?? and a photo would be nice. May be water washed something.

 

What happens when you try to start? Cranks or not?

If it does, you may like to Search for
bore wash