More than likely you have a faulty or mal-adjusted safety switch. These are fitted to the blade engage, brake, and possibly the transmission gear shift (if applicable). Their purpose is to prevent starting in an unsafe condition.
Check if battery voltage appears on the solenoid trigger wire when switch turned to 'start'. If so, the solenoid is either faulty of not grounded properly. If not, check those switches.
phil_saunders(a)bigfoot.com
Phil Saunders
May 2009