"Strike gold" but without the implication of searching?
Solution 1:
Among terms for a stroke of fortune are godsend; stroke of luck; lucky break, strike, or stroke; fluke; windfall; fortuitous. These words are used in phrases like "What a godsend she has been!" and "That really was a lucky break for you." Less directly applicable are out of the blue, pot of gold, unlooked-for.
Solution 2:
The word that comes to mind to describe this is serendipity:
serendipity, noun : the faculty or phenomenon of finding valuable or agreeable things not sought for; also : an instance of this