If its giving a warning before it starts, it usually means you have a short or bad sensor or bad sensor connection. Check the fuses. You may have gotten a wiring connection wet, or they may be corroded from the long term storage. I'd pull loose as many wiring connections you can get to and spray them with some dielectric anti-corrosive spray then re-connect them a couple of times to polish the connector pins. Be sure the battery is fully charged (should show 13+ volts), and check the connections to be sure there is no corrosion, and they should be real tight (wrench, not screw-driver).
Then try again. There should be another symbol lighting up on the dash to give an indication of where the problem is, or a two digit error code. Check to be sure the impeller has not locked up.
2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100