I had a peculiar issue today where I suddenly lost the ability to see any bluetooth devices on my Linux Mint 18.2 desktop utilizing a Plugable USB Bluetooth adapter.

The fix involved checking kernel messages for anything insightful. In my case this is what led me to the solution:

[ 608.988353] Bluetooth: hci0: BCM: Patch brcm/BCM20702A1-0a5c-21e8.hcd not found
[ 609.156320] Bluetooth: hci0: BCM: chip id 63
[ 609.172330] Bluetooth: hci0: LPP-3389-WIN
[ 609.173313] Bluetooth: hci0: BCM20702A1 (001.002.014) build 1764
[ 609.173347] bluetooth hci0: Direct firmware load for brcm/BCM20702A1-0a5c-21e8.hcd failed with error -2

After some googling I finally came across the solution here. The fix is to download the firmware for your bluetooth adapter and place it in the place the bluetooth kernel module expects it to be in, then to reload the bluetooth kernel module.

sudo mkdir -p /lib/firmware/brcm
sudo wget https://s3.amazonaws.com/plugable/bin/fw-0a5c_21e8.hcd -O /lib/firmware/brcm/BCM20702A1-0a5c-21e8.hcd
sudo rmmod btusb bnep bluetooth btrtl btintel bnep btbcm
sudo modprobe btusb bnep bluetooth btrtl btintel bnep btbcm

That did the trick! You can also reboot your machine instead of removing / re-loading the kernel modules and it will accomplish the same thing.