Default Galaxy S3 I9300 Stuck on Samsung Logo

Hi guys,
I'm sorry if I have posted in the wrong section but I'm trying to fix a friends phone and no matter what I do I just get stuck at the Samsung logo with an error in the default recovery screen about E: unable to mount or something like that.
Is anyone able to help me step by step to fix this.
He never rooted his phone this just randomly happened.
I'm sure I have all the files needed for the repair but after trying so may of the fixes I have read online I am now lost.