Hi, Ellie.
On most Samsung cameras, the NX300 included, this means that there's a connection problem between the camera and the battery; usually the result of dirt in battery chamber or on the battery's contacts that prevent the camera from receiving the proper voltage needed to fully operate. Start by powering the camera off and removing the battery. Next, make sure that the contacts on both the battery and the camera are clean and in good state. You may use a cotton swab dipped in rubbing alcohol to clean the inside of the battery chamber. Once everything is good to go, carefully replace the battery and power the camera back on.
Should there be damage on any parts of the battery, consider acquiring a new one. You can find additional support on some common battery problems on the NX300's [[Samsung NX300 Troubleshooting|troubleshooting page]].
Hope everything goes well!