Diagnosis
That's normal behavior, not a fault. This model doesn't have a backup battery to retain memory. When it loses all power (from a breaker trip or unplugging), it reverts to a default state, just like when it was first installed.
How to Fix It
There's no fix, only resetting. After power comes back, you just need to reprogram the clock and any settings you use, like the child lock or favorite cook times. Press the 'Clock' button, enter the time with the number pad, and press 'Clock' again to set it.
If That Didn't Work
If the clock is resetting even when the microwave hasn't lost power—like it randomly goes blank while the kitchen lights are still on—then you have a loose wire connection or a failing main control board. That's a different and more serious problem.
Heads Up
If you're going to be working on the microwave for any reason (like changing a light bulb or cleaning behind it), expect to reset the clock afterwards. It's just part of the process with this unit.