Spacecraft that orbit Earth from the upper atmosphere, like the International Space Station, encounter slight amounts of drag that slow their trajectory down over time. That said, most spacecraft have limited lifespans after deployment. Engineers can combat said drag with thrusters, but fuel sources only last so long before depleting.
Given the circumstances, what goes up must come down (eventually), and space agencies are responsible for ensuring that spacecraft don't enter Earth's atmosphere over highly-populated areas. This prevents super-heated chunks spacecraft metal from landing on people and causing severe injury or death.
On the other hand, China lost control of its Tiangong-1 spacecraft long ago, and experts have warned that it would re-enter Earth's atmosphere sometime between late March and early April of 2018. Well folks, the calendar is knocking. Experts say Tiangong-1 will re-enter Earth's atmosphere any day now, and we still have no idea where it's going to land.
Sounds scary right? Maybe just a little bit, but experts are staying positive about the situation by citing Earth's vast water-covered surface as being an excellent shock absorber. Given that two-thirds of Earth's surface is water, it's unlikely that any piece of Tiangong-1 will strike solid ground. Even if it did, the chances are slim that fragments from it will smack you in the head.
Experts are continuously observing the skies to spot and keep track of Tiangong-1 as it enters Earth's atmosphere, but it hasn't yet. Everyone's crossing their fingers and hoping for the best.