Another member and I both had skis doing this exact thing and it's pretty tough to troubleshoot without a wiring schematic and a pretty good grasp on electrical systems. Thought this might help somebody else.
Here we go.....

This will apply to most, if not all, '07-'09 4tecs with 4 WIRE DESS POST. Symptom will be no power to anything. No start, no beeps, no display, just NOTHING!
Obviously, you need to check your battery voltage and connections.
Check the connections at the starter solenoid.......many times somebody replaces a solenoid not realizing the system power wire needs to go on THE SAME post as the battery cable, NOT the starter side.
Sometimes you find a bad fuse, or even the main power relay, which sits by itself along with a couple 30a fuses.
If, you've already checked all these things and still yet nothing happens, try this.......
Access the backside of the main power relay panel. You will find a few red power wires and also a few black ground wires. Add a jumper wire from the relay ground straight to the battery negative (ground) terminal. You should immediately hear the relay click AND the gauges might even power up. Try starting it. If it seems that everything works normally shut it down and remove the ground jumper wire. We know at this point that we are NOT getting a ground signal to operate the main power relay........WHICH COMES FROM THE DESS POST, and ONLY there. There is no other ground to that little panel. I assume there is an extra reed switch inside the 4 wire post which goes OUT to the black wire with purple stripe. This ground path is supplied from the ECU just like all the other sensors and electrics. If the start button is pressed, OR lanyard is installed, the ECU then sends the ground signal OUT through the DESS post and on TO the main power relay panel allowing full power up.

I spent 5.5 hours tracking this down and fixing it, the other member got hit about $750 at the dealer. Just thought I might save the next guy some're welcome.