I've had trouble with my Sigma 19 mm lens not responding when attached to my Sony mirrorless cameras. I was concerned that this was a warning of imminent failure, but, to date, cleaning the contacts has cured the problem.
I've tried cotton buds and that appeared to do the trick for a while, but after the latest episode of non response, this was not enough.
As a measure of last resort, I used the buds in conjunction with some Eclipse E2 sensor cleaning fluid. This worked, and saved me the cost of a new lens - I believe that Sigma regards these budget lenses as being disposable, and don't offer a repair service ( please correct me if that is wrong!).