Et alors ? is commonly used in that case.

Beware that it must be asked an interrogative/ascending way like in the Henri Salvador song “Zorro est arrivé” (thanks to 変幻出没 for his comment) otherwise it means “So what?“.

Alternatively, you might also use Et puis ? or Mais encore ? as Graffito suggested.

I don’t really agree with jlliagre‘s answer. In my opinion, “Et alors ?” often means “What you just said is useless”.
I agree “Et alors ?” can be used in a positive way, but my point is that it may be confusing.

I would use one of those instead :

Oui… ?
Ensuite ?
Et puis ?
Et… ?
Allez, accouche ! (slang, saying you can’t wait anymore !)
Vas-y, accouche ! (slang, saying you can’t wait anymore !)