今日は休みのはずだったのに、急に呼び出された。
Q. What actually happened to the speaker?
Translation
Today was supposed to be my day off, but I was suddenly called in.
Explanation
はずだった + のに signals that an expectation ('should have been a day off') was overturned. 呼び出された (passive) says they got summoned — so the day off didn't happen.