I changed my gamer tag and now it made me start a new game and I can’t drive the cars I already own. I still have all the titles I have earned I’m still in the club I’m part of and I can use the cars I shared but they are not in my own garage. I had to start at square one again. What happened and how can I fix it
Not sure what went wrong there, you should not have needed to start a new game. I changed my GT as did many of my friends without any issues. Sad to say, if you’ve already started a new game it has probably wiped out your old one, and you will need to do it all over again. 
If you did not change your old gamer tag to the new one, then the old profile should still be there.
Check with Xbox Live support at www.xbox.com
Well my game is still on my hard drive bit I can’t access it