I am not a dell tech. I am just here to help others with my knowledge.

Step-1: Bluetooth service

Check if bluetooth service is running.

1. Press Windows + R to launch the Run application. Type “services.msc” in the dialogue box and press Enter.
2. Once in Services, navigate through all the entries until you find “Bluetooth Support Service”. Double-click the service to launch its properties.
3. 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. 

This just happened to my new Dell laptop after turning it on from Hibernate. Tried restarting Windows but no luck. What fixed it was performing a full shutdown. To do a full shutdown in Windows 10 press the shift key and select Shut down from the menu. After turning on the computer, the Bluetooth icon is back in the Task Bar and in the Action Center.

https://www.isumsoft.com/windows-10/2-ways-to-perform-a-full-shutdown.html

Mine is also having this problem now. First time.. Still going through windows update, hopefully that does it. I didnt see any bluetooth info while in safe mode.. I do hope there is a more permanent fix and soon..

Encountered this same issue with my DEL 7490. Restarted computer few times but didn't help. Performed shut down and then power on and problem was fixed. Probably when Restart function is used, computer is not fully turning off.

Thanks Ande for the advice.

My Inspiron 5770 has had the same disappearing bluetooth problem ever since I got it. I even got Dell to replace the wireless/bluetooth card and still the problem persists. 

My bluetooth disappeared again at the start of the week and I assumed it would come back some time or other when it felt like it. However, as you recommended, I just rebooted in Safe Mode and then returned to a standard reboot and voila - Bluetooth is back :-). If someone wants to explain to me why this has happened, I'm all ears.

Jonathan