Leonardo uploading troubles

Good evening!
I have a confusing trouble with my arduino:
I bought it, tried it, had a few uploadings, everything ok; but a few days later, i tried uploading an improved version of my program, and then it stopped working :
It is not detected anymore by windows when plugged, but it is when i press the reset button. (appears a few seconds)
It seems that this problem occured when :
-i changed my wiring (but seems ok, and anyway arduino's are protected from short circuits, no?)

  • i used the Serial commands (basic)

I thought my arduino was broken, and so i changed it; but i tried same thing on my new Leonardo, the problem is the same.

To specify my problem:
-wire checked and OK
-tried on Win7 and mac OS, same problem
-no short circuits or hardware misuse
-"read-first" forum messages checked
-drivers manually installed

lukanio proposed me a solution, but it seems not to work.

Thanks for your help!

Any idea please? Thanks!