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Make sure to be within the device's Bluetooth range
Maybe your device's Bluetooth feature works, but your device is out of range of the other Bluetooth device. The range can vary from 3 to 100 metres, depending on your device. If you are not sure what range your device can cover, search for its bluetooth specifications on the Internet.
If you cannot pair your (smart)phone or tablet with another bluetooth-capable device (e.g. computer, tablet, bluetooth speaker/headphones, etc.) restart the mobile device. After it turns on, you should be able to pair it with another device, provided that its bluetooth feature works.
If you cannot pair your computer with another bluetooth-capable device (e.g. smartphone, tablet, bluetooth speaker/headphones, keyboard, etc.) restart the computer, after which you should be able to pair it.
If your device is powered by non-rechargeable batteries and you were unable to turn it on, consider replacing the batteries. To be 100% sure that the batteries need to be replaced, test them on another device which uses batteries of the same type.
If you paired your bluetooth headset or speaker with your computer or mobile device, and everything goes smoothly but you can't hear any sound, check if you have accidentally activated the mute option. All you have to do is to unmute your device and you should be able to hear sounds coming out.
Contact the device manufacturer's customer support
Maybe your device's bluetooth issue can be resolved easily by the manufacturer's customer support team. All you need to do is to call them and follow their instructions. Also, most of the manufacturers have a troubleshooting page on their websites, which can help you fix your device's bluetooth feature by yourself.
Bluetooth devices cannot communicate unless they are paired. In order to pair devices, you need to enable the Bluetooth feature on both of them. Then, go to the bluetooth menu of one of the devices and through it select the other device and tap on the "Pair" button. Then, follow the instructions you will be given.
Go to the Bluetooth menu of the computer or another mobile device and scroll down to see the list of paired devices. Locate the device you have issues with (e.g. bluetooth speaker, headphones, etc.) in establishing Bluetooth communication and remove it from that list, by tapping on it and selecting the remove (delete) button. Then, initiate the pairing process again and follow the instructions.
If this method didn't work, check out the other solutions.