My watch won’t charge up and is just showing a blue triangle

I would try doing a hard reset of the watch - My Garmin Fitness Device Appears to Be Frozen or Locked Up

Then, if still a problem a factory reset, which would typically be doing this - Resetting All Default Settings

However, since you cannot get past the blue triangle, you can factory reset by using the Unprompted Reset method.

• Ensure the watch is powered off - You may need to hold LIGHT for up to 30 seconds to turn the watch off.
• Press and hold BACK and START/STOP.
• Press LIGHT to power the watch on while holding these two buttons.
• After the first beep release START/STOP.
• After the second beep release the remaining buttons.
• You'll know if the reset was successful if the watch finishes powering up and goes through initial setup.
• After reset, place the watch outside with a clear view of the sky for a minimum of 20 minutes to acquire satellite data.

If after all of this, there is still no joy, contact Garmin Product Support.