If something happens serendipitously, it happens by a lucky chance or by accident.
Serendipitously is a happy and lucky word. It means something good happened without planning.
Examples:
She serendipitously found her favorite book at the library.
I serendipitously met my best friend at the park.
The cat serendipitously found a warm spot in the sun.
I serendipitously discovered a secret treasure map in the attic.
Serendipitously, I won the lottery and became very rich.
The balloon serendipitously floated down and landed right in my hand.