How can i fix my car Renault megane 2001?

My Renault megane automatic gearbox light is on in the dashbord the car is not changing gear when driving it stuck on probably 4 or 3 gear.when i have to take off it taking time but driving once it start moving.the light (Auto) is on in the dashboard.i changed the oil i put new oil but the problem still.please help i don't really have money for a new or used gearbox to fix and i unable to go to work untill i fix.
I'm very desperate.thanks for helping me