Why does my battery-SLA not have any power?

Why does my Sealed-Lead Acid (AGM) not have any power?

The battery may be old or depleted. On 375, 750, and 1250w inverters; low voltage<11v or not reading at all. The battery cable may have been attached to the wrong terminals and damaged the inverter. Check the datecode and replace if needed. Check volt reading on meter. If <11v, recharge the battery with a separate minimum 10 amp trickle charger back up to 11.5v. Replace if needed. If issue persists, contact Newcastle Technical Support. http://www.newcastlesys.com/submit-trouble-ticket