The Nintendo DS lite powers on, but the screen is blank..

I'm sorry to say this, but if by 'card reader' you mean Game Card Slot, then your system is modified, and in the process of doing so, the person 'fixing' it damaged it worse. It is now no longer fixable by Nintendo, and the system is will continue to be a black hole of money for you if you continue trying to fix it.

I recommend cutting your losses now and buying a new system.

"An Error Occurred. Press and hold POWER to turn the system off."
When do you see this error?

1. If you see this error while playing a Nintendo DS game card game:
   There are no known issues that create this error with any Nintendo DS game card games. Our experience shows that if you see this error, your game is likely a counterfeit product. To test your Nintendo DS game card, try this testing tip:
   
   
   
   • Try playing the game in a Nintendo DS Lite or a Nintendo DS (original style).
     • If the game still does not work, it's more likely the game card is defective and will need to be replaced.
     • If the game works in either a Nintendo DS or Nintendo DS Lite, our experience shows that the game is likely a counterfeit product.
   
   
2. If you see this error when there is no Nintendo DS game card inserted into your system:
   • If you have not already done so, power the system off and then back on to see if this resolves the issue. If the issue persists after the system has been powered down, please call our Consumer Department at 1-800-255-3700 to speak to one of our representatives.

Hi are you talking about playing ds games or gameboy advance games

possible that the flex holding the lcd is broken.

Last time I had this I had to change the motherboard, tried for hours to find the fault. motherboards are about £50 if you have to resort to it. Make sure you are buying an original nintendo and not a copy

There might be something (dirt or dust) blocking up the ds card slot. So try blowing on the card and in the game slot.

You can play multiplayer games with a Nintendo DS Lite even if you don’t possess a copy of the game. If one of your friends has a multiplayer game card, they can use their Nintendo DS Lite as a host for the game session and you can join the multiplayer game.
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Things You'll Need:

• Nintendo DS Lite

Using One Card to Host the Game
1. Step 1 Look on the game’s packaging for the "Multiplayer" icon. If the game requires only one player to have a copy of the game, the icon will show two DS Lites: a DS Lite with a black screen transmitting waves to a DS Lite with a white screen. Below the two DS Lites is a range of numbers which indicates how many players the game can support.
   
2. Step 2 Put the game card in the Nintendo DS Lite that is acting as the host for the multiplayer game session and start it up.
   
3. Step 3 Have the other players turn on their systems. They will use the main menu screen to access the "DS Download Play" icon. They need to tap the icon to begin downloading the game information from the host DS Lite.
   
4. Step 4 Begin playing when all of the systems have finished downloading the game information from the host.
   
Multiple Cards for Multiple Players
5. Step 1 Look at the "Multiplayer" icon. If every player in the game session needs a copy of the game in order to play, the icon will show two DS Lites, both with black screens, transmitting waves toward each other. A range of numbers is under this graphic and indicates how many people can play at one time.
   
6. Step 2 Read the game's instruction manual on how to set up the game session. The steps required vary by company and sometimes by title within the same company.
   

*reposted from ehow.com

I had bridged the F1 and F2 fuses and then replaced the top screen and seemed to fix everything

if you need more info or are looking for someone to repair I also do console repairs and have fix quite a few nintendo ds lite systems.

let me know and ill help you out.

either info or a fix

Do you know anyone else who has a nintendo ds (doesn't have to be lite)? If so, try the game in that sytstem, and if the same thing happens, then the game has a glitch. This tends to happen a lot with plenty of my handheld games, and i find that it helps if you blow into the game cartridge, and also into the cartridge hole. If that doesn't work, try repeatedly turning the game on and off. And if that doesn't work, try going to nintendo.com.

If you see lines on either screen of your Nintendo DS Lite during game play, then it is likely one or more of the components that create the image on the screen are damaged. This can occur if there has been an impact -- such as being dropped or hit -- to the Nintendo DS Lite's screens. Phone 1-800-255-3700 to discuss available repair options and they will be grateful to assist you at any time of need.Important Note: If you are seeing lines on the screen that slowly fade away after turning off the system, this is normal.