@swanlefitte pointed you to the perfect thread:
Pentax Lens 18-250 Smc. Lock button jamed - PentaxForums.com
I just tried it. My 18-250 locked up 3 years ago and I was unaware of the trick. I gently overturn the rubber cover of the lens, using a smooth non-sharp spatule (a stapler remover in my case). I found the screw - It is not at the location of lock. I used a tiny watch screw driver - I had to use a magnifying glass to help me find the right tool and to lock back the screw. My DA18-250mm is back in business (alongside my DA18-270mm.)
Great, neat and simple trick.