2004 Chevy Impala 2004 Impala wont start sometimes

Please help me. My 2004 Chevy Impala, 79000 miles, has been showing the security light for a while. It has also been not starting sometimes and after I let it sit for a while then it will start. Sometimes I have to let it sit overnight. The feul guage and speedometer will not reset to 0. I am at a lost because when I start the car everything comes on- lights, radio, heat works, everything except the engine will not turnover. What could be the problem. I even has the battery checked. It has a full charge and I don't think it's the starter. Please help me??