Is it a Combo unit, just sonar or just GPS? If you have not already done so, Pull the power plug from the back of the unit and check for power in the plug. If a combo unit or just sonar the power plug will be the one for the transducer. Make sure power is present in the positive socket and that the plug and pins are not corroded and are making contact. Reverse the procedure to find out if the ground side of the plug is ok. I had a similar problem and found that the plug was bad, locking ring did not engage the grooves as it was twisted/turn into the locking position allowing for a poor contact.