I have a solid white triangle on my forerunner 55

Maybe try a hard reset?

• Hold POWER BUTTON on for 15 seconds. The device should hopefully start to turn off.
  
• Press and hold UP ARROW BUTTON and DOWN ARROW BUTTON
  
• Press but don’t hold the POWER BUTTON for a second or so – device should turn on
  
• Wait for 1st beep then release UP ARROW BUTTON
  
• Wait for 2nd beep then release DOWN ARROW BUTTON