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Welcome to the Fitbit community, @CoombeT!
Sad to hear that your Sense isn't working as expected.
As there is the option that your device's battery is completely drained, please have a closer look here to make sure it is able to get charged:
Why isn't my Fitbit device's battery charging?
I'd like to point out that it can take a while for your device to start charging if the battery is on deep discharge.