Actually you can access the battery, but it is a bit complex (compared to other phones).
It is possible it isn't the battery, and it is the charging cord, that is the issue. Have you tried the cord with another device to see if it works? Also, try a different cord on that phone to see if it works.
If the cord turns out not to be the issue, check if the phone is still under warranty. If it is then you should contact the manufacturer about a repair or replacement.
If it isn't under warranty anymore, then you will need to either do the work yourself, or take it to a local tech/repair shop and have the battery replaced.