Steering trouble

Did you hit anything, kerb/pothole etc?  
 
Check the obvious, loose wheel nuts, bent struts/suspension arms, tyre pressures, steering ball joints, steering rack mounting, suspension joints, tightness of all suspension mounting points, alignment adjustment bolts etc.
 
You might also be able to see one (or more) of the wheels being out of alignement if its quite bad.
 
Also check wheel bearings.
 
Cheers

Sounds like something slipped or broke.
If you haven't already done it, park the car on a level surface with the wheels straight ahead. Stand a few meters or so out the front, and look to see if there is anything obvious, like excessive camber/toe etc.
 
When you raise the car, check all suspension parts that arty mentioned,  and a good idea to check the rear of the car as well.
 
I once hit a bad pothole in my sdub, then noticed the steering wheel spokes were not level. Turned out that the strut/hub bolts had slipped, causing bad negative camber. (that also caused bad toe out).
 
 

Yep. As reddtarga said, I meant everywhere. Even fitting one rear wheel with different offset causes the car to veer in one direction, especially under power. This is what I experienced when I got a puncture in the back tyre and borrowed another wheel.

If yours is not self centring that would point to castor though would it not ?

If its such a significant issue it should be visually obvious.

Cheers
Cheers