First, Take a deep breath and relax! Not that it matters much but; Is it the mode selector on the camera body or the switch on the lens?
If it was not happening before: Are you using a new bag or strap? Does the switch feel loose?
If none of the above, you must be inadvertantly touching the switch when you pick up the camera. Pay attention to how you are picking it up or holding it and you should be able to prevent it from happening again.