Идиома означающая pleasant surprise

Гудбай америка о.

Gobsmacked ?

Godsend

сон в руку

В отрыве от контекста их может быть много. Рискнуть предположить, что это lucky break - идиома, которая в словаре Коллинза трактуется как a fortunate and unexpected turn of events (с пометой British English).
That was my lucky break.
They admit they are now desperate for a lucky break.
The second lucky break for Gough came midway through last season.
I got a bit of a lucky break.

Есть также интересное слово - serendipity (good luck that makes you find something interesting or valuable by chance - счастливая случайность). Примеры: The scientist's discovery was a real case of serendipity.
It was pure serendipity that we met on that trip and became lifelong friends.

Есть выражение knock somebody's socks off. Пример: The food at the newly opened restaurant absolutely knocked my socks off.
Идем дальше.

• Hit the jackpot - неожиданно получить большую выгоду или преуспеть в чем-либо (часто ассоциируется с азартными играми). Пример: She hit the jackpot when she found her dream house for half the price.
• Бритицизм a turn-up for the books (something that is very surprising and unexpected, and usually very pleasing - CollinsCobuild). Пример: `This is a real turn-up for the books for me,' he chuckled, leaning on his bike at the finish. `I've never won a race before.'
• Fall/drop into one's lap - нежданная удача. Пример: The chance to go on a trip to Baltimore dropped into my lap quite suddenly.

be on cloud nine - быть на седьмом небе (от счастья)
A purple patch - Полоса везения, удачи