Start with a basic "walk-around" inspection to check for any obvious problems such as loose wires or cables, visual damage to the unit and appropriate fluid levels.  If your generator still won't start, check for these common issues:
1. Generator switch is not turned on
2. Choke knob is not out 
3. Generator switch is not in Choke position (INV model only)
4. Fuel is contaminated (Look for water in your fuel tank - this means that the fuel has deteriorated and must be drained).
5. Spark plug requires replacement.
6. Battery needs replacement (12kW model)
7. Engine Idle Control (12kW) or Engine Smart Control (INV2000) should be in the Off position
8. You are attempting to start the generator under load.
9. The generator has shutdown on overload and must be reset to OFF postion (INV models only) 
If none of these solutions fix your problem, call the Customer Interaction Center or take your unit to a local Caterpillar Home & Outdoor Power Authorized Caterpillar Sales and Support Center (ASSC).