Vivoactive 4 has a blue triangle when powering on, can’t get the watch to start at all on holiday and access to a computer to connect it to .. any ideas ?

No access to a computer to connect to 

...usually the situation you described with the blue triangle indicates damage to the system memory - you need to contact Garmin support....((((

If you have the charging cable with you, turn off the watch, and  press and hold the upper button while you connect to power.  Release the button after about 15 seconds.

This worked for me! Thank you for the tip.

That didn't help me. Now the sheen is black with little white dots. It will no longer power on with the cable plugged into it or not. So frustrated. This is the second Garmin that's done this same thing.

I had this happen to me before. The first time I got the blue triangle, my phone stopped working. It would not power up. The second Garmin have me the blue triangle but when I held the power button wn for 15-30 seconds the screen started with squiggly designs then the screen went black with white dots and powered off and powered off and never came back on.  So sas.