Press Control-Open Apple-Escape, all at the same time. Use the up arrow key to highlight "Control Panel" and press return. When you see the Control Panel use the arrow key to move up to "Slots" and press return. Look at the bottom of the screen to see which slot is set as the "Startup" slot. If it is set to "Scan" or "Slot 6" then there is a problem with either your drive or the drive controller card. If it isn't set to "Scan", or Slot 6, then use the up arrow to highlight "Startup"and then either the right or left arrow key to set it to slot 6, or Scan. Press return, then scroll down to "Quit" and press return. Scroll down to Quit again and press return to get back to the main screen. Turn your GS off and then back on, if you have a disk in the drive it should start the drive up and boot the disk.
Dean