So your baby finally started sleeping longer and you thought you were in the clear? Well, unfortunately most infants have a relapse in sleeping habits that is commonly known as the 8 month sleep regression. This is nothing to be alarmed about, but you will have to deal with another 3 to 6 weeks of unsettled sleep habits until your baby settles back down again.
Sleep regression is generally assumed to be a natural outcome of your baby being up and more active during the daytime as it starts to explore its world. As the baby starts having its present sleep habits, including daytime naps, disrupted, that disruption flows over into its nighttime patterns as well.
There is not much that can be done except to care for your infant and wait for them to grow out of this natural progression.