why the ministry didn't punish the Malfoys for following Voldemort?

Narcissa lied to Voldemort in the Deathly Hallows when she berkata Harry was dead...that kept them out of Azkaban after the Battle of Hogwarts.

I guess they were afraid of voldemort and everything was in chaos once the death eaters took over

'cause the Ministry is stupid

lucius malfoy donates a lot of money (that is mentioned in HP5)

Lucius was punished, he was in Azkaban. Narcissa was'nt really a death eater she just went with it because her sister, husband, and son were. And Draco I think he was forced to be a death eater and didnt want to be.

After the first fall of Voldemort?
sejak my calculations because Malfoy Ministry bribe! So Lucius as the ministries in charge! : P

If after the saat fall of Voldemort, I know they have been indicted in Azkaban, but I'm not sure! : (
anyone can tell me?

I think it was because the Ministry realized that, in this case, they'd already suffered enough.