You may have a bouncy push button or you may have inadvertently double-clicked the reset button, as some boards have a "double-press" feature which causes the bootloader to be entered. This will result in the LED "breathing" on and off around once per second. Usually if you wait about 10 seconds, it will time-out and go back to normal operation, but some other boards may be stuck in bootloader until you press reset again.
You may have a bouncy push button or you may have inadvertently double-clicked the reset button, as some boards have a "double-press" feature which causes the bootloader to be entered. This will result in the LED "breathing" on and off around once per second. Usually if you wait about 10 seconds, it will time-out and go back to normal operation, but some other boards may be stuck in bootloader until you press reset again.