I went back home last month and put my Charge HR in a corner since then. But when I prepare to wear it these days, I found my Charge HR shows a battery symbol with an exclamation mark on the screen. It cannot display dashboard as usual, neither can I connect that device from my smartphone. My smart band was fully charged, so this issue was not caused by lack of power.

Two circumstances may cause this problem the battery may be depleted if you have not use your smart band for a while. To solve this error, hold the button of Charge HR for over 5 seconds to perform a reboot.

Or else this problem occurred during a firmware update. To solve it, you need to download and install the Fitbit connect software (PS: DO NOT download that Windows10 metro version, it does not work.)on your PC, then click “Check for device update” button on the software frontpage. When the update is done you’ll see a confirmation message and your tracker will restart.

ref:

• What does “!” (An exclamation point) mean? – Fitbit Community
• Solved: Battery Symbol with Exclamation Mark – Fitbit Community