Alarm LED
The ASA 5500-X performs autonomous environmental monitoring, polling all external sensors and monitoring operating conditions. In the event of damage to certain internal components, or surpassed temperature thresholds, the system activates an alarm LED to notify you of a critical condition. For example, the alarm LED is activated by firmware in the event of various critical over-voltage and over-temperature conditions, as well as when the ASA has missing or unrecognized internal chip components. When the alarm LED lights, you can find details about the system condition from the system message that appears on the console, or by entering the show environment or show controller pci CLI commands.
Is it only the console that doesn't let you log in or is there no function at all? Power off the ASA, turn power on again and see if there is any output. Regardless of what happens, open a TAC-case for support/replacement.
