Sketches won't upload to Uno
You can set the verbosity level in the Arduino IDE:


File menu
Preferences
Settings tab
"Show verbose output during" => check "upload"
Click OK


Upload again, and note the additional output telling you what is wrong.

It should also point you to this URL, which points out the most occurring issues.

The issue I have most, is that I forget to switch between Micro and Uno board (as I deploy my sketch on both boards).