Toyota Repair: Fuel system, toyota camry, gasoline engine

Question
I have a 2001 Toyota Camry 4 cylinder.  I was wondering what would happen if the fuel pressure is not properly relieved (i.e. the harness connecter underneath the backseat is not removed) prior to changing the fuel filter?  Could it damage the fuel system (like the fuel cannister) or cause the check engine light to come on?  My check engine light came on immediately (within 1 driving cycle) after having the fuel filter changed and I am trying to figure out why.  

Answer
No electrical connectors need to be removed when replacing the fuel filter, may be coincidental, have the diagnostic code checked and let me know what the code is and I can go from there. By the way is this a CNG(natural gas) engine or or regular gasoline engine?