Please post the exact model number of your system. Without that a diagnosis is impossible.

Step-1: Bluetooth service

Check if bluetooth service is running.

Press Windows + R to launch the Run application. Type “services.msc” in the dialogue box and press Enter.
Once in Services, navigate through all the entries until you find “Bluetooth Support Service”. Double-click the service to launch its properties.
If the service is stopped click restart. And also select startup type as automatic.
Step-2: Booting in safe mode

I am assuming you have win 10 installed.

open start >> press power icon >> holding shift press restart.

After restart do the following.

troubleshoot >> advanced options >> startup settings >> restart.

After restart press 4 to open windows in safe mode.

Check if bluetooth is present in safe mode.

To exit safe mode simply press restart.

Some users found that booting into safe mode and back to normal mode solved their problem,

Step-3: Windows update

Sometimes windows update might fix the problems. Try updating windows by settings >> update and security >> windows update.

Step-4: Diagnostics with support assist.

Run a hardware scan and support assist app will let you know if there is any problem with your bluetooth driver/hardware.

If it says it's all fine, then open troubleshooting tab and under system devices click bluetooth device.

After troubleshooter check if support assist shows any errors.

Even after following the above troubleshooting steps didn't solve the problem let me know.

Check Windows Device Manger (You will need Administrative rights to run Windows Device Manger.) and make sure that all Bluetooth devices are enabled and none are disabled. I've disabled the Wi-Fi chip driver for good reasons and because I disabled the Wi-Fi chip. The option to use Wi-Fi was nowhere to be seen. In order to have Wi-Fi options I had to reenable the Wi-Fi chip driver. Once I did that then I use Wi-Fi like normal. I'm telling you this as something like a Bluetooth chip driver being disabled, and as such you cannot use Bluetooth. 

What is happening to my Dell latitude 4301? The Bluetooth suddenly disappeared