As far as my understanding of current science goes, you can't "develop" ADHD as an adult, it is something you are born with or perhaps develops at a young age. Perhaps if you had some kind of traumatic brain injury, which may be what you're alluding to.

Either way, first step is go speak to a Psychiatrist about it. You might need to get a referral from your primary care physician. They will help you figure out what treatment options are available to you.