How can I fix peugeot 307 "automatic gearbox faulty"?

hello people , i have peugeot 307 automatic , i have problem with my gearbox , when it need to change from 2. to 3. gear , on display apear automatic gear fault , and it stay in 2.nd gear and in the next period that don't change gears , i must stay and agin start the engin to restart gearbox ... so what is the problem , ehat i need to do ??