Thank you for your reply. But the problem is not with my computer but with the board. trying a different port and on another computer did not resolve my issue. It really seems that the upload of the multiserial example had gone wrong for a reason, it seems the connection between the board and the computer might be responsible. and because of that my board is no longer programmable,I paid a lot, regarding to what i am earning, to get this board and I really need it for an important personnal project. I have no tool to test or reflash the bootloader but i have a few spare arduino nanos, could I use them to reflash the bootloader, using arduino as isp?
update: after searching for a while i found that it is not possible to reflash the board usin arduino as isp, as the mkrzero is an arm board and the arduino an avr one. Well it seems that I am doomed with my no longer functionning board.