1. Try other charging cables, and use an 1-2A adapter to charge it for 1 to 2 hours.
2. Leave it aside for half of a day.
3. Clear pairing (Keep the ear hooks inside, and then long-press the multifunction button of the charging case for 12s after opening the case lid.)

4. Use alcohol wipes to clean the four charging contacts detailedly, and put UTWS3 back to the charging box when the contacts are dried. If the indicator light is not on still, please repeat 2-4 for a few times.

5. If the above measures cannot help that the UTWS3 still cannot be searched for by mobile phones, it is suggested to ask for after-sale service.

You could also read the following video for help: 

What to do if the UTWS3 can not be charged or the indicator light keeps off: https://youtu.be/cm8xeG35Hrw 

How to clean the charging contacts of FiiO UTWS3: https://youtu.be/y0pQr4qeqY4

How to adjust the curvature of the ear hooks before putting it back into the charging case of UTWS3:https://youtu.be/OWHTGqfH7OM