I'm not familiar with Yamaha keyboards, but here are some general steps you could try.
0) (Obvious): Make sure the master volume and any other hardware volume controls are turned up.
1) Try testing all the different outputs in both mono and stereo (I'd even suggest checking the R on it's own for any L (MONO)/R pair), including the headphone output, if there is one, and test with 1/4" cables and an amp you know are working. Doesn't matter if it's your bass amp or something that you'd never use with a keyboard; all you care about is getting a sound. The first thing you want to be sure of is that there's no cable or amp problem and that it's not a problem with a specific output.
If nothing works, leave a 1/4" in the main mono output and run into your amp for all the other tests; simpler is better until you figure out what's wrong with it.
2) If you have access to another keyboard and the W5 has MIDI input, get a MIDI cable and rig the two together (other MIDI OUT -> W5 MIDI IN). Make sure the other keyboard is sending on MIDI channel 1. Try to see if you can get any sound out of the W5 that way; could be that its keyboard is defective. If MIDI channel 1 doesn't work right away, find the way to set the W5 to receive on MIDI channel 1 and/or get the other keyboard to send on each of the 16 MIDI channels, one at a time, and see if that fixes things.
3) See if there's a factory reset that restores all settings to their defaults. If so, do that. If you can do that and still get no sound, take it in; it's probably an issue with one of the circuit boards or one of the outputs; it's obviously not an issue with particular settings.
4) Try cycling through a large number of presets and see if they're all silent. If so, see if there's a volume level for an individual preset and if so, pick a preset and adjust its volume to full.
Can't think of anything else; this basically covers in-memory settings, outputs, cables, amps, and keyboard. If none of this works, you're probably stuck with taking it in for repairs, or at least an estimate.