The computer repair shop cheated me (opens in new tab)

After I got home, now the computer has a problem every time it starts up three times. But clicking and loading content (such as entering a password) are extremely slow. Sometimes even after entering the password, the computer will directly go into a black screen, with only the mouse pointer visible. All the RGB lights and accessories of the computer are on, but I can't control them.So what should I do? Does anyone know exactly what's wrong with the computer?