This could either be a software error or hardware error.
If it’s a software error, try to perform a RAM reset by removing the batteries (including the backup battery) then reinserting them.
If that doesn’t help, try to reinstall the OS. This is more complicated so I’ll refer you to a video tutorial [https://youtu.be/8HlPgS0elRM?t=10m22s|here].
If neither solution worked, refer to [[TI-84 Plus Silver Edition Troubleshooting#Section_Buttons_are_not_responding|this guide]] for some hardware troubleshooting and fixes