I can't unlock my old iPhone?

Last Christmas I got a new iPhone 5c so I put my iPhone 4 just locked up in a cabinet but my girlfriend has a crappy LG so I was going to give her my old phone but when I got it out it said disabled and won't let me do anything! Is it because I haven't used it in a while or what because I thought if you got a new phone the carrier just switched over but you could still use it like an iPod touch kind of? So any advice helps, please and thank you!

Try a code.