A guild gets deleted when there is less than 3 people online in 15 days. So to make sure a guild never gets deleted make sure you have more than 3 people online at least ever 14 days. (For example. I log in at least every Friday for 15round just so my guild does not get deleted. If I miss 15round I try to do NT. If I miss both I try to log in at least 3 then log out.)

There is a max amount of guild required per server. This makes room for other guilds if some remains inactive they will be dismissed so that it's possible for new guilds to be created in the future.

Aniways you should have gotten a message in your inbox no? saying why the quild was disbanded. (due to inactivity, the leader disbanding it or less than 15 people in guild)

Look here to learn more about guilds.
http://wl.igg.com/guide/guide.php?acid=189