Hi. My name is*****'m a camera repair technician with over 20years in the business and I'll be glad to help you with your question today.
Is the aperture control ring set and locked at the smallest aperture?
One option that may work is to clean the contacts on both the camera body and lens with alcohol and a cotton swab. Try this and let me know what happens.
Do you have another lens to try on the body to see if you get the same result? This will narrow down the malfunction to either the 70-300 lens or the camera body itself.
At least you know the problem is in te lens and not the camera body. These lenses are subject to flex cable failures which will need to be installed by a technician. On the other hand, it may just be a loose connection to the circuit board but it will need to be disassembled to troubleshoot and repair. This really isn’t a user serviceable problem and should be repaired by a qualified technician. You may want to check locally for a camera repair shop in your area, or, ifyou need assistance finding a shop, please let me know and I'll locate one for you.