USB drive not showing on my Mac after using it on Windows I have noticed this on my pin drives and also my external hard desk. They were perfectly working fine on my Mac Pro, and then after using them on my friend's Windows they don't show up again on my finder, and also they're not found in desk utility. What to do?
I've had this problem and its common.  I would like to help but will need more information about your attempts to solve because there is more than on potential solution.

Restart your computer.  Sometimes that works just like a nap will make you feel better.

Check the power adapter (try a different one but be careful to pick the exact same voltage and cable) and trying different USB cables.   

Drive format/file system issues are the next best guess for why its not working.  I'm sure you didn't reformat the drive for Windows when you used it on Windows because if it was reformatted then it will need to be reformatted again and the data is probable gone and while you could pay for data recovery its highly unlikely that would help.

Drive letter issues is another.  Open Disk Utility and see if the drive is showing up there and if it is run First Aid on it and see if it starts showing up in the file system after you do that.

To use it for both you need to format it for FAT32 or exFAT.  exFAT enables larger file sizes relative to the older FAT32 which caps files in the 4GB range but exFAT only works on newer computers with more recent Windows XP and/or Snow Leopard (10.6) or newer operating systems.

For more information, you can try this more comprehensive guide: http://www.online-tech-tips.com/computer-tips/external-hard-drive-not-recognized/

Hope this helps!  With more information I could help!