if you already did it several times, then this time your flash failed because i.e. image was not downloaded correctly or something like that.
If you really can't get into failsafe mode, then you have to unbrick it via serial access to the bootloader and initiate a TFtP upload
If you really can't get into failsafe mode, then you have to unbrick it via serial access to the bootloader and initiate a TFtP upload