Starts half the time. runs strong when it starts. cranks when it doesn't start. replaced worn spark plugs. started 3 times, ran great. now won't start. Any help would be much appreciated!

 

likely that crank position sensor probably was on way out, now is dead. Check for spark at all plugs. If you have none, replace crank position sensor. If you do have spark on one or more plugs, let us know which and will take it from there