Li-ion Battery Protection - Recharging
The protection circuitry disables the cell output, to protect it from deep discharge, which is damaging to Li-Ion chemistry. If you start charging it, and if the cell is not completely dead, it should be fine. 

BTW, if you discharge the cell below protection threshold, and then disconnect any load, a normal healthy cell will re-bound by itself to some degree, slightly above the cut-off threshold.