Answer to: Start Button not workingTry taking a 2HB pencil over the pad on the membrane to redeposit graphite. If that doesn't work, try looking at the start button's electrical traces to see if there's a break. If there is, you're gonna have to sand and solder in a bypass to make it work again.
Answer to: lines on screen, no pictureCheck your capacitors, they may have blown.
Answer to: Why my brightness isn't working?Your capacitors are probably gone and have leaked everywhere on the board. It's best to get a game gear capacitor kit and replace all your mainboard caps (sometimes your audio caps too, however the power caps are generally fine). Be warned though, it does involve soldering and cleaning.