Our son has a 07 Accent, went to work day before yesterday and car started and ran just fine. Yesterday went out to start and go to work and wouldn't start. Once key is in ignition and turned the insturnment panel comes on, the radio works. Thought it may be battery, had it checked and is great, cleaned the cables and check the wires then plugged back in. Changed the starter and the problem continues. When attempt to start, nothing...no crank, no noise, nothing.....need assistance, what could be the problem

 

There is a starter relay in the engine compartment.
Try .. Switch it with the horn relay and then try to start the car
If it starts then the relay is faulty.

If it doesn't start then try to bypass the ignition switch by putting a jumper wire from the S post of the starter to the Battery B+ positive terminal.
This can also be done at the relay if you know which two points are for the control circuit and which two are for the starter circuit. Jump the starter circuit.
If engine cranks then it could be an immobilizer or bad ignition switch or short or open somewhere
If it doesn't crank then should check the electrical grounds for the engine
The starter uses the engine block as a ground.
If the grounding wires or straps are bad then the starter will not turn because there is no pathway for the current

Edit: don't forget to inspect the socket for the relay.
Good luck