Why always my battery power percentage shows 0%?

Today my PC's charging percentage shows 0% always. Even I plugged in charger for hours. When I unplug the charger the screen is going dimmed. What is the problem?

when charger is unplugged

when charger is plugged

I'm surprised a machine reporting zero charge keeps running even for a few minutes when unplugged. As suggested in comments, this may be a hint that your battery is about to completely fail, but it might also indicate that your battery capacity calibration has been lost.

The usual way to recalibrate battery capacity is to run the machine on battery until it shuts down due to lack of power (disable any power management shutdowns for this), then plug it in (without powering up) for longer than it should need to fully charge (overnight will usually do it, without any power going to run the machine). Be aware that if the battery is on its last legs, this may finish killing it; this deep discharge might prevent the charge circuits from charging if it's below 1.0 V per installed cell.

This might fix the calibration; if it doesn't, you should probably try to at least check the machine with a known good battery to see if it's a sensor issue or possibly a bad cell.