I had a similar issue, try this
You may have dirt under the monitor unit, easy fix. I get this 3-4 times a year
from rain etc.
Remove unit from handlebar mount. The monitor slides out and may be by a plastic clip, bend down, you'll see it.
CLEAN the connections on the back of the monitor, it may be 2 little metal dots.
Clean off connections on mounting unit that is on the handlebar.
A bit of alcholol on a paper towel or just water.
Re-Mount, Spin wheel and see if it the screen indicates speed.
if it does, you're done. If not try again.