My galaxy S7 active shuts down after startup

The phone just freezes up after startup,it will run through the Samsung start up and Att,to the lock screen pattern.I enter the pattern,and sometimes it will go farther,other times it just freezes.The phone was a gift,it was bought refurbished.It had a 30 day warranty but on about the45day it went down.Is there anyway to fix it ???Answers please.