After I filled up gas I turned on my vehicle, and as soon as I shifted out of P, the shifter was loose and I couldnt get it to lock at D. But when I turned my engine off, it landed on R and has stayed there. I can't get it out of R. Luckily it is able to reverse ,so I reversed my vehicle into a stall.

 

The bush on the end of the selector cable that goes onto the gearbox is known for failing.