Diagnosis
This is the default power-on state. The microwave has lost its memory for the time, usually from being unplugged or after a brief power outage.
How to Fix It
You just need to set the clock. Press the Clock/Kitchen Timer button once. Use the number pad to enter the current time, like 1-3-0 for 1:30. Then press Clock/Kitchen Timer again. The colon will stop flashing and you're good to go.
If That Didn't Work
If pressing the buttons doesn't respond, the control panel might be locked. Press and hold the Stop/Clear
Heads Up
Yeah, this one's common. A lot of folks think it's broken, but it's just waiting for you to tell it what time it is. It needs the clock set before most cooking functions will work.