My 50mm 1.8 AF-S is having issues focusing... It is hit and miss if it's focused. I will spend lots of time making sure it's focused and the focus indicator light comes on, but the image won't be focused anywhere. However the next picture may be. It's almost like it's getting major sunflare, or like it's focusing on an invisible point in mid air, thus throwing everything out of focus.... Help! :-) It's my most used lens!

SOURCE: The Auto focus just stop working

Hey Sammymalone,
This lens should have a 1 year manufacturers warranty on it, and if my guess that the USM motor is burned out is true I would have it serviced by an authorized service technician. Before you do this though I would try cleaning the lens contacts to make sure it is communicating with the camera body. I of course am going on the assumption that you have tried other lenses on the camera body to make sure the problem is the lens and not the body. I hope this helps!

Sincerely,
Allan
Go Ahead. Use Us.

SOURCE: auto focus not working

Your Sigma 28-300mm doesn't have a HSM-motor as far as i know. When you switch it from MF to AF, you also have to disengage the manual focusing ring by pulling it towards the camera, or pushing it away from the camera. Using the AF without unlocking the manual focusing ring may damage the AF motor. 
I hope this helps, 
/Chris

SOURCE: canon ef 50mm 1.4 can't focus <1m

sorry, your autofocus engine might be broken or damaged. ou will have to take your lens to a repairman

SOURCE: auto focus not working

Sorry but if you've tried all the usual fixes then there's nothing more that you can do. Either pay for a professional repair or continue to use the lens manually.

A professional repairer should be able to indicate what the likely costs are going to be and you can then judge whether repairs are cost effective.

Personally, I'd stick to manual focus as I've always preferred knowing exactly what I'm focussing on instead of constantly fiddling with buttons and menus to override what the dumb camera has locked onto.

SOURCE: AF-S DX VR Zoom NIkkor 55-200mm f/4-5.6G is not focusing

Your motor may have gone bad. Try sending it to PhotoTech Repair Service in NYC. They are Nikon authorized, and can do the repair under warranty if you still have it. Also if you join they're facebook page, they will give you a 10% discount.
Here is the facebook page link, it has everything you need to send it in.
http://www.facebook.com/pages/New-York-NY/PhotoTech-Repair-Service-Inc/102527215194

Testimonial: "Thank you for your feedback. I suspected that might be the issue. Thanks also for the recommended repair service."