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Restart your mobile device
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.
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.
Enabling and disabling the Airplane mode is like a soft restart of the device's components, one of which is the Bluetooth feature. Before you do anything else, enable the airplane mode and disable it after a few seconds.
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.