What can I do to get my Sigma 75-300mm telephoto lens to fit my Alpha mount Sony? The lens will attach but the camera says "there's no lens". I used this lens on a 7000 Maxxum 35mm film camera.
The problem you are experiencing is due to the way third party lens makers usually design their lenses to be compatible with cameras made by someone else: they reverse engineer them to work on the existing offerings of the camera maker at the time they design the lens. If the manufacturer later updates their system the original manufacturers have access to the actual design parameters and can usually create an update that remains compatible with their older products. The third party lens makers such as Sigma do not have the same access to this data. As a result, many time older third party lenses such as your Sigma 75-300 will not be fully compatible with newer versions of the system they were designed for in years past. Sometimes they won't work at all.
I have an older Sigma 70-300mm lens that fully functions on my old Canon film EOS camera from the mid-1990s. When I attach it to my newer digital EOS bodies, however, it is severely crippled. If I shoot in manual exposure mode and select the widest aperture setting available so that the lens need not stop down I can take a manually focused picture (even though the same lens will auto focus on the older film body). If I try to use any Av other than wide open it causes an error that will not allow the camera to take a picture. I then have to power cycle off and back on to reset the camera and remove the error condition.
In the past Sigma has at times offered to re-chip their older lenses when an OEM camera maker such as Canon or Nikon releases newer cameras that make the older Sigma lens obsolete. The new chip contains updated firmware for the lens that (hopefully) allows it to be fully functional on the newer cameras. At times Sigma has done this for a limited amount of time at no charge. After the "free" period has passed they would typically then do the update for a fee. Eventually the service for a particular lens is dropped as the demand for the update subsides. In the case of your 75-300, the lens is probably not worth the cost of what it would take to ship the lens to a Sigma authorized service center combined with the cost to have them re-chip the lens if Sigma even still offers the service on such an older lens.
And face it, lenses offered today, even at the low end of the consumer level, are far better than low end consumer lenses such as the Sigma 75-300 were a couple of decades ago. It's probably time to update your telephoto zoom lens!