iPhone freezing after operating system upgrade

it all started when my front facing camera stopped working and has ended with my iphone freezing whenever it wishes to - sometimes i can get into emails or make a call but other times the slide doesn't move to unlock or if it does it freezes so i can't access or do anything. it doesn't even ring! and it happened, i believe after the guy at the cell phone store upgraded to 8. no one has been able to help me and i am truly sad because i love this phone and i can't afford a new one. i have tried to do a factory reset and it didn't work. help?