Why is my samsung galaxy s6 stuck in reboot mode?

my samsung galaxy s6 is stuck in reboot mode it will load up to the samsung logo then cut off right away then it will load up again then cut off again and it will do this over and over again and has been doing this for over 10 hours it has a rattling noise whenever i shake it and at times it will heat up till i can barley touch it the back of the phone is also cracked but its been cracked for almost a year now. I've tried rebooting factory resetting all of that so i need to know what to do or who to go to that will fix this cheap?