uhm, did you check the position of the magnet on the spoke? and/or the little recepter thingy bopper on your front fork (gosh I'm so technical)?

if either one gets moved then no speed shows or wildly innacurate.

on my Cateye wireless if it shows time/date but not speed and/or distance that's it and a simple teeny tiny tweak wll do.

When the battery dies the whole thing's greyed out.