I can not make personal ringtone for my phone as it is not visible as portable device in 'My Computer'. My operating system is Windows 8.
Did you make any changes before the issue occur?
You can try to restart the Phone and the PC and check if it helps.
If the issue persist, you may try the Tips for solving problems with USB devices:
Also there is a material about how to add or create ringtones, more information see:
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