The headset may only need to be reset.
A hardware reset of the headset can be accomplished by a simultaneous long press of the black POWER/MUTE button and the SPEAK VOLUME DOWN (_) key. Then let the headset rest for 1 minute before powering it back on. More detailed instructions are available here
Additionally, if you are using the 510S system with your office phone, then the headset may have lost its pairing to the base. Please see page 15 of the attached Users Guide for instructions to resubscribe.