my keypad is having trouble why?

My family moved in this house in 2001 and the alarm system was already here so I dont know what kind it is,.. and lately people have been creeping around the house and things have been getting stolen so I was already worried. Then today I found a window open (in the winter!!) and none of my family opened it. So I set the alarm and went to bed,...I woke up a lil after midnight and forgot I set the alarm to motion detect the hallway and I set it off. I put in the code and it wouldnt shut off. I practically sprained my thumb pushing it in so hard like ten times then having to go to the second key pad and pushing it in six times before it finally disarmed,..its an older keypad so maybe thats it,....if not why else could it be doing this and how can i fix it??