Start by checking all of your fuses. Then if they are ok, you will need a good automotive electrical diagnostic person to dig deeper.
This just happened to me. It's apparently a common problem with the factory stereo on '03-'06/07 G35 models. Like Answer #1 says you can see if it's a fuse problem. Also, a small section of VIN numbers do have a recall on the factory stereo in case this problem occurs so you can check to see if your car was covered and you can get it fixed for free. For me, none of those options worked and the infiniti dealer in South Jersey quoted me at $550 for the repair. It's normally more, but he said something about "since it was a common problem, Infiniti gives that flat rate now." I went a different route and installed a double din aftermarket radio/dvd player for close to the same price.