Why is the screen blank with vertical lines?

It sounds like your GBA SP may be experiencing a problem with its display. Here are a few steps you can take to try and resolve the issue:

1. Clean the cartridge slot: If the cartridge slot is dirty, it can cause connectivity issues between the game and the console. Try cleaning the slot using a cotton swab and rubbing alcohol.
2. Check the game: Try using a different game cartridge to see if the issue persists. If it does not occur with another game, the issue may be with the original game cartridge.
3. Adjust the brightness: The GBA SP has a brightness control dial that may need to be adjusted. Try adjusting the brightness dial to see if it resolves the display issue.
4. Replace the screen: If the above steps do not work, the display screen may be faulty and may need to be replaced. Replacement screens for the AGS-001 model are available online and can be installed with some technical know-how.

If none of these steps work, it may be best to contact a professional repair service to diagnose and fix the issue with your GBA SP.