8 year old cat is terrified and dont know why

Question
Me and my parents have had our cat now for over 8 years and he was one of the most friendly cats in the world but 2 days ago he changed and now is terrified of anything and this has become very distressing to me and my parents as we love him so much.
We have had 3 cats previous to our current male cat and never seen this happen before. He comes home to eat but the moment he hears anything he disappears and we do not see him again for a number of hours and then it repeats when he eventually returns.
He lets us stroke him and comes up to brush against us but suddenly out of no where he changes and runs off.
We have no idea what to do.

Answer
Harry,

Sudden behavioural changes can indicate illness. I would have a veterinarian evaluate your kitty as soon as possible to determine if he might be ill...If you have any further questions or concerns please don't hesitate to contact me again at any time - I'm more than happy to help in any way that I can.

Kind regards,

Ali