Diagnosis
A flashing 'M' light is the desk's way of telling you it's in a programming mode loop or has a fault it can't clear. It's confused about its memory settings.
How to Fix It
You need to force a full factory reset. Here's how: 1) Unplug the desk from the wall. 2) Wait a full 60 seconds—this lets the internal computer fully power down. 3) Plug it back in. Immediately after plugging in, press and hold the 'M' button and the 'Down' arrow button together for about 5 seconds. You should see all the lights flash. Release the buttons. The flashing 'M' should stop. Now, test the up and down arrows. It should move.
If That Didn't Work
If the flashing persists, the issue might be a stuck button on the hand control. Carefully inspect the 'M' button. Does it feel sticky or depressed compared to the others? Try gently pressing around its edges to see if it pops out. If a button is physically stuck down, it will keep triggering that mode.
Heads Up
After this reset, all your programmed height settings (M1, M2, etc.) will be erased. You'll need to set them up again by moving the desk to your desired height, holding 'M', then pressing and holding a number button until it beeps.