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=MvDN0UQSlug
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.