Assuming you are sat on the mower, out of gear, blade clutch out, brake full on, and still nothing, then you have either a faulty safety interlock switch (likely), a dead solenoid (possible but unlikely), or a faulty key switch (very unlikely but possible).
You will need to check if battery voltage appears at the small solenoid terminal when the key is turned to START. If it does, the solenoid is faulty. If not, check if battery voltage appears on the START terminal of the ignition switch when turned to START (access may be a problem, but it is possible). If not, key switch is faulty. If it does, then you have an interlock problem. Replace or bypass.
phil_saunders@bigfoot.com
Phil Saunders, May 2008