Is your keyboard and mouse still not functional? Have you tried to replace batteries? You can also try to connect a wired keyboard and mouse.
It is difficult to perform any diagnostics of the system if keyboard and mouse are not responding.
If you gain access to these input devices, you can run diagnostics from the Dell site.
https://www.dell.com/support/product-details/en-hk/product/dell-ebt2250-desktop/drivers