I've recently had an issue with my DC where the laser wouldn't read any games (I guess someone was touching the laser, definitely not me). I fixed it by opening up the Dreamcast and turning the orange screw in the back of the laser assembly a tenth of a clockwise rotation. This increases the power of the laser to make it read games better. If this doesn't work, you could try cleaning the laser with a cotton swab with a dab of rubbing alcohol on it.
You may have to end up replacing the laser or the entire assembly though. There are tutorials for that, but it does require a bit of soldering.