I have baked cakes for quite sometime and all of a sudden they have started to crumble when cutting. Any ideas

Ok, I know nothing about making cakes, but am trying to think up a valid reason why someone's cakes might start changing.
Your oven is no longer working as well, affecting cooking time and eveness?
You changed cake tins affecting cooking subtly through heat absorption and distribution?
You are using a different brand of flour/butter/margerine/olive oil spread/other ingredient which might have a different grade/consistency/water content/something else?
You have become impatient and don't wait for them to cool properly?

Does any of those suggestions give you any ideas for a reason?

Chill the cake for a few hours before slicing- or you could try increasing the egg content of the recipe.