Check for plugged screen/filter. 

Check for air lock on gravity-fed systems.

If your pressure washer pump is starved for water it will lead to serious internal problems. 

Check here for correct hose I.D. and water flow.  

Look for a plugged screen or filter.  

Click here for a discussion on water filters.  

Check here for selection of replacement filters.  

Purge air from intake hose for gravity-fed systems.  Most pumps will not draw water and need the suction flooded on start-up.

Check here for discussion on various types of pumps.