I have a 2005 G35 coupe with about 90000 miles .

Notices recently that my starter was going out or so I thought . Took it to the mechanic and he confirmed that was the case . He changed the starter and on my way home I went to get gas and when I went to start the car and it wouldn't turn over . I called him he met me at the Gas station and took a lok at it and said it was the alternator now. He said that is how it ussually happens , when one goes out the other is close to follow . So , he replaced the alternator . I go to start the car this morning and nothing the car won't turn over again. Help what else could it be ? Obviously this mechanic doesn't know .

Remove and fully charge/replace the battery. Most auto parts stores will test and charge batteries for free.

If I'm right, and it's the battery, insist the mechanic refund some money. Otherwise, that's repair work at its worst.