Existing iPad trying to activate unsuccessfully

While charging the battery, my iPad automatically went into a screen to activate. It was unable to activate and generated an error message indicating it was unable to be activated because the activation server is temporarily unavailable. It suggested retrying in a couple of minutes or connecting to iTunes to activate (neither worked) or to contact Apple Support. 1) why did it try to activate (my iPad was running up-to-date software, and isn't new-was working fine prior to charging) and 2) how do I fix this?