the yellow triangle is just a general failure on the device. it can be anything really.
follow these steps to enter recovery mode. https://www.youtube.com/watch?v=MvDN0UQS...
from there, you can ractory data OS reload. PLEASE BE ADVISED: this will wipe all of your data on the device. you will lose photos, contact, videos, and any other personal files.
if this doesnt work, i usually find that this is commonly a battery. however, it can also be a board level issue.
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