Calibration mode is ended when powered down.
I would even go so far as it does not store new data if not ended properly, so by turning off the old, wrong settings were still in use.
The flashing LED may be either Overtemperature (red), no signal (green) or overvoltage (red/green).
I think the XL5 has a "off center throttle startup" protection, too, which will flash the led. In this mode the ESC does not accept a wrongly pulled trigger on startup to prevent runaway.
Maybe you moved the 50:50/70:30 throttle switch, maybe you maxed the throttle trim, maybe your trnsmitter is broken and makes a weird signal (loose pot or something).
Or the XL5 went wet and wrongly detects something, something i had on my stampedes XL5. There i had problems reversing because the shorted coolers made it think the motor still turned.
So by calibrating you set up the throttle detection, which made the off center protection go ok, and that solved your problem.