Firstly, don't automatically assume they want to break-up with you. When someone's behavior shifts in a relationship, it shows that something is off, and it needs to be identified, worked-through, and discussed. 

Get honest. If things have deteriorated, you have nothing to lose by getting things out in the open (which is most likely what should have been done long before this time). 

Get involved. If one or both of you have dropped back, reverse your course. Get your minds and efforts back in your relationship. Sometimes a close-call of breaking up is just the thing a relationship needs to wake everyone up.

Say what you need. If you (or they) are dissatisfied with how things are going, talk about it. They can't read your mind. The only way to get things better is to tell them directly and explain why you are unhappy. From there, both need to put their efforts towards each other to reestablish the connection.

Bottom line: If you are noticing things that are off in your relationship, don't just wait for them to bring it up for fix it. Take action.