2007 malibu, driving 65 mph, power steering warning on dic, error on odometer, speedometer quit working, transmission downshifts rpm to 5000 (i guesslimp mode),shut off in drive way, now won't start.

What do I try next? You take your vehicle to a qualified repair shop. You have no idea of the complexity of the electronic's on your vehicle . Disconnecting battery cables an touching them is not the answer . Having a qualified professional with proper diagnostic tools checking computer system's is . One bad module on the GMLAN serial data network is all it takes for a no start . You have no idea what happens when you turn the key to start your vehicle.

Circuit Description
For ignition switch power modes refer to Body Control System Description and Operation. Once the ignition is placed in the Run/Crank position, the control circuit of the Run/Crank relay is grounded by the body control module (BCM). With the Run/Crank relay switch closed, battery positive voltage flows through it and on to the park/neutral position (PNP) switch. With the PNP switch in either the Park or Neutral position, battery positive voltage will flow to the starter relay coil supply voltage input terminal of the engine control module (ECM) and the coil side of the starter relay. Placing the ignition in the START position sends a message to the engine control module ECM requesting engine start. If the ECM has determined that the transmission is in Park or Neutral and theft is not active, it will ground the control circuit of the starter relay. Battery positive voltage will then flow through the switch side of the starter relay to the S terminal of the starter solenoid, cranking the engine.

Also with power steering warning on ,DTC'S are stored in the control module for the electric power steering an or the BCM - body control module . I have seen bad power steering control modules short out the data bus (network) .
Do yourself a favor an take it to a qualified repair shop.