Hi. My name is*****'m a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
This is caused by a damaged or defective flex cable connecting the power diaphram to the main circuit board in the lens. These go bad all the time and require replacement.
This really isn’t a user serviceable problem and should be repaired by a qualified technician. Your best bet would be to check locally for a camera repair shop in your area. If you need assistance locating a shop near you, please let me know and I'll locate one for you.
I do go to a camera repair place but they tend to charge the earth, which is why I came on here. Now that I know what the problem is I can make sure that is all they repair and not find other 'faults'.
Thanks for your help.