Probably the magnet that has become lose and has locked the voicecoil.
Sadly, that's very common, especially if dropped or shipped with inadequate packing.
Try pushing the cone inwards (gently! and don't push on the center dome) and feel if it
will move or stands completely frozen.
If frozen, find a good original woofer and replace.
If it moves, the problem lies elsewhere, maybe a lose wire or a defective crossover filter.
Martin