Just spray a small amount of WD40 onto the game cartridge's connectors. Insert the cartridge into the ds lite and then remove. Use a toothpick to clean between the pins inside the ds lite and you should be back to normal.
I just fixed this problem on my daughter's ds lite. The pins were not bent, but it was not reading any games. Spray a small amount of WD40 onto the game cartridge's connectors. Insert the cartridge and remove. Then use a toothpick to clean between the pins on the ds. Back to working oder!