I had few problems with uploading sketch to Arduino on win10.
There are two option to try for fix...
1.) Run IDE as Administrator - it worked for me first time but then it again stopped working.
2.) I don't know if it's some win10 update, but from yesterday night I had problem with uploading in one moment. I then found, that avrdude is not able to start properly.
Problem is in library libusb0.dll. To solve this problem, simply download libusb from here: Download libusb-win32-bin-1.2.6.0.zip (libusb-win32) extract libusb-win32-bin-1.2.6.0/bin/x86/libusb_x86.dll and rename it to libusb0.dll. Then copy libusb0.dll to C:\Program Files (x86)\Arduino\hardware\tools\avr\bin
Good work finding the dll solution! That one had the Arduino developer confused for a while before he figured it out.
There is a bug in Arduino IDE 1.6.10(and possibly Arduino AVR Boards 1.6.12 used with any IDE version) that can cause that issue when you start the IDE by opening a sketch(.ino) file(by double clicking on it, etc.). You can also fix it by doing this:
Close all Arduino IDE windows
Start the Arduino IDE, not by opening a sketch file.
Open the sketch file via File > Open, File > Sketchbook, or File > Examples
The last opened sketch will automatically load when you open the Arduino IDE, that one should also work fine.