Below is the answer from Cat Eye's website. 9 times out of 10 the magnet has moved either up the spoke or the front wheel was put on with the magnet on the opposite side from the sensor. After that the sensor could of moved outboard a cm or 2 from the magnet. Then start with the battery tests as outlined below. Remember with a wireless model you have two batteries, one in the computer and one in the transmitter.
From Cat Eye's website http://knowledgebase.cateye.com/questions/67/How+do+I+get+my+computer+head+unit+to+register+a+signal+with+the+sensor%3F
How do I get my computer head unit to register a signal with the sensor?
If you computer head unit does not pick up a signal or does not seem to "communicate" with the sensor - check the following:
make sure the "AT" icon or "AUTO" is displayed on the screen (computer
is in auto-timer mode, not manual stop/start mode).
sure the magnet is passing within 2-3 mm of the mark on the sensor (on
the fork blade or chainstay, depending on model).
If neither help restore a consistent signal, then it could be either a low battery or a damaged wiring (Bracket/Sensor) kit.
if the LCD display is clear and bright, the battery may be too low to
transmit a signal from the sensor to the computer head unit.
with a fresh battery installed, try performing a manual restart
operation to bring the LCD display back to life. Do this by either 1.
pressing both buttons (Mode and Set on back) OR 2. all three buttons
(Mode, Start/Stop, and small Set button on back) at once and then
releasing all three OR 3. by pressing the AC (all-clear) button if your
computer has one. This should bring the display back to life, and the
unit can now be re-calibrated for your bike. See if consistent signal
Finally, If that does not help, take a paper
clip and bridge the 2 diagonally opposed metal contacts on the back of
the computer head unit. If random information appears in the current
speed display (upper portion of screen) while bridging the contacts,
then the computer head unit is fine and the problem lies with the
bracket/sensor kit. It may be stretched or damaged just enough to
interrupt the signal transmission, even if not visibly so. If that is
the case, please visit our small parts site
to purchase the appropriate replacement sensor kit.