+1 no punishment
+1 on chatting
Your daughter sounds almost like the perfect daughter! I wonder if this is big a problem as you're making it ?
Did you ask her why she bombed? Maybe, she had a fight/misunderstanding/etc ... with BF.
Let her tell you what is up. Jeez, don't we know by now that kids know better than the parents

Seriously though, let her do the talking. You might find out the real reason and from there you can guide her.
Now for grades:
I agree they are important
but :::
They seem to be only good for to get into a good university and to get a good first job!
( A good job can mean many things ... this situation refers to what many may call a "steady, fast track position, that takes care of you for life

) My opinion ... may differ and should go in a new thread
What else are grades good for????
- A house loan?
- A good BF/GF >> partner?
- A job after quitting the first one?
- To become the POTUS ?
I bet there are many P owners that worked they way up in the world and grades had very little to do with it.
Dueller, you're another good PPOT dad, so don't worry, just sit down with her and show that you are on her side and that you are just worried about her in an experienced adult way and yes, of course that you are her dad, a dad that wants to be on her side