If you forget a language you used to know fluently, could you regain your fluency?
It probably depends on why you lost fluency. If the brain is injured, it will likely be possible for some but not for others.
Otherwise, I would imagine the answer is yes if you have the time, energy, motivation, and resources. It might even be easier to regain fluency than for those who were never fluent and started from scratch. :)
As for my experience (though the language I am trying to regain is one where I wasn't exactly fluent), you are going to have to put a lot of work in it for you to regain what you had of use in it. It's not going to come back to you magically, though it will surprise you how much you remember of it.