If your power bank is flashing 2 lights, it's likely that the power bank is being charged with a wall/car charger that transfers less power than required for your particular power bank.
Many of our larger power banks (eg. 10,000mAh or higher) require a minimum of a 2AMP wall charger (see packaging or manual of your power bank for clarification).
You can usually find the AMPs of your wall charger printed on the charger itself.
Once you have plugged your power bank into a stronger charger, leave it there for 8-10 hours. It may flash 2 lights for the majority of this time but it will eventually return to normal.
For even larger power banks, you may need to leave the power bank charging for longer, see this table for more details.