Thanks to all who replied. It seems that not keeping the battery
charged over the winter was the main cause. I initially jumped the
battery using my car and let the engine run for an hour, but I still
couldn't restart it afterward, which led me conclude the battery was
dead. Over the weekend, I used a charger to charge it at 1.5A for 12
hrs and 0.5A for another 12 hours. It does seem to be back to live
now. I'll be more attentive to keep it charged in winter months.