DSi wont read the games you put into the top, but the bottom is fine.

I have had this DSi for around 6 or 7 years, and about 4 years ago (when it was 3 years old) it stopped reading the game cards I would put into the top. The bottom still worked fine, still does, but the top would constantly say "No game detected." or something like that. Anybody know what could possibly fix it? I am prepared to replace parts. I can't sell it though. Help is appreciated!