2006 Pontiac G6 Unhooked Pos Battery Cable Resulting in Pro

My car's low oil indicator wouldn't reset and in my experience with older cars in the past unhooking the pos bat cable for 30 seconds and putting it back on would reset the computer. After I did this when I would turn the key in the start position the doors would lock unlock fast and repeatedly everything on the dash was flashing on and off. Everything just went nuts and It wouldn't start. We called a friend of ours who's a mechanic and he said that I should have taken the neg off first then the pos and connected it back neg first then pos and that If I unhooked both cables and let it sit for about 15 min my computer would go into rest mode and should be fine again. Well this didn't work and my grandfather told me to try charging the batter so I did and that's how I got the car started. Now my CD player wont work and it makes weird almost computer type sounds randomly and my 6 disc CD changer which is full says there are no discs and if I try to do anything it says " CD changer error ". I would like my cd player to work again. Esp since this is such a new vehicle but I'm also more worried something more serious is wrong. Do you know what I did wrong and what I might try to resolve this issue? Any help would be greatly appreciated. Thanks! ~Adam