Depending on when people ask, a system may go down due to it needing upgrades. When something like the playstation network needs upgrading, the only way to do this is by shutting down the system so that no one can use it and conducting the upgrades. When the upgrades are finished, then the users will be allowed back on the network to use it again.
The most recent time for the playstation network to be down was from January 17th – 19th. Usually this maintenance does not need to take place for several days. Instead, it usually only happens for a few hours. A message will be displayed on their website so everyone knows to prepare for when the playstation network will be down.