Check the Connection
Go to the back of your unit and ensure that the power plug is fully inserted. It might help to unplug it and plug it back in. A simple restart should be your first step.
Try this step with each connection point of the wires going to your fish-finder. It could just be a case of the plug slipping out a little bit, but that may result in problems with the device turning on.
Fish-finders usually require 12 volts for full power. If your battery level has dropped to below 12 volts, the fish-finder might not turn on because of an insufficient charge.
Charge your battery or replace it if necessary. Make sure it gets up to 12 volts. Then try to turn on your fish-finder again.
Another issue that can cause problems with your fish-finder not turning on is any fuses connected to it. These could be on the device itself. They could also be wires attached to the battery or fuse panel.
Loose or Bad Wiring
Wires and their connectors are other areas to check. If you have a poor crimp connection or a frayed wire, it might cause your fish-finder not to turn on. Follow all wires going to your fish-finder. If there are areas sealed with a crimp or other connector, consider replacing that connection point.
Test the Unit
If possible, take your fish-finder unit to another boat or battery setup. Test it on the wiring of that separate boat or directly to the battery. If the fish-finder works, then the issue is related to the power supply in your boat. It could be the battery, fuses or wiring.