how to fix a window what as come off rail on Citroen c5 2008 diesel 1.6 ?

Only a guess as they are similar in design.
Had same on Peugeot Partner combi which is basically similar to design of citroen and you MIGHT need new actuator unit.
Check with citroen service garage. Or even contact citroen. www.citroen.co.uk

Then again may be different that ball joint on raise/lower arm has pulled out of the runner slide.
Not an easy job but doable. Being old and decrepit had to give mine to garage

Bob uk
February 2019