How long have you left your headphones charging for? Often times if a device has not been used for a while, the battery can go below the recommended discharge voltage and not turn back on.
That being said I would recommend that you leave your headphones charging for 12 - 24 hours before using them again. This will give the battery enough time to properly charge back up to the voltage it needs to be at.
Please keep me informed and let me know if this ended up helping you, or if you need further information. Hope this helps.