There are a couple of conditions in which an ASA 5500-X series will not automatically come back online following a power cycle.
#1 is if the initial shutdown was due to environmental issue (overheating).
#2 is when there is an extended period of outage (e.g. 24 hours). In those cases, some earlier manufactured units may require a manual push of the front panel power button to be restored to service. See this thread (and this BugID).
Short duration power outages should result in the ASA automatically coming back online.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...