Hi Giorgia,
It sounds like Windows has lost track of your Bluetooth adapter and is treating it like a generic USB device. Start by shutting down completely, unplugging power for a few minutes, then booting back up to see if the Bluetooth branch reappears. If not, open Device Manager, uninstall the “Unknown USB Device,” and then use “Scan for hardware changes” to reload it. If the toggle is still missing, grab the latest Bluetooth driver package from your motherboard or wireless card manufacturer and reinstall it. If drivers don’t bring it back, the adapter itself may have failed, so testing with another OS or a replacement dongle will confirm whether the hardware is still good.
Domic V.