You were right Wuerfel_21
I got the COM port back (without microSD card) after double-clicking the MCU reset button.
The conclusion would be that, if you have your Yun expanded with a microSD card, after reflash the OpenWrt-Yun image (U-boot, kernel and rootfs), and before expand again you would have to:
1- Format you card. I used SD Memory Card Formatter for Windows/Mac | SD Association
2- Expand http://www.arduino.cc/en/Tutorial/ExpandingYunDiskSpace
Thanks