Posting your code properly in tags (control-T in the ide to get snazzy indents first, then control-shift-C to save for forum, control-V into post) would make it a lot easier to read, but I'm not seeing where you save the current state as previous. Might be missing it though...
edit: see the state change detect example in the ide (File > Examples > 2 Digital) and here. (It has the pin wired to ground, note.)
It's always a good idea to test concepts (like checking the state change of a pin as here) in code of its own so that the other stuff in the sketch (all that matrix stuff here) doesn't get in the way. Then put that tested and understood new concept into the code at large.