Question

After the oven pinged to let me know that it was hot enough. <----how do i fix this fragment?

Answer 1

The word "after" seems unnecessary.  If I had to rewrite that sentence I would say :  The oven pinged to let me know that it was hot enough.

Answer 2

Afterwards, the oven pinged to let me know that it was hot enough.
OR add more to the sentence
After the oven pinged to let me know that it was hot enough, I placed the bread inside.
OR get rid of the source of the problem
The oven pinged to let me know that it was hot enough.
It really just depends on the context of the sentence.