When I help someone search for their bio parents using AncestryDNA, I start by asking them a standard list of questions. At first, people may insist they have no information at all, but I seldom find this to be true: most people have more information than they think. Even with a closed adoption, most know where they were born, and when they were born. You may just need a little help pulling it out.
The first two questions I ask are: Where were you born? What year were you born? Just the where and when can be tremendous help.
When it comes to a single bio parent search, which is almost always a search for an unknown bio father, family members may have given you more information than you realize: Did they ever drop a name? Do you know where you were born? How old was your known parent when you were born? What places have they lived? Do you know their ethnicity? Have you asked aunts, uncles, cousins, grandparents, parent’s friends if they have any of this information?
Try to put together the scraps of information you’ve picked up over the years: Your mother was born in 1960 in Toronto; you were born in 1985 in Saratoga Springs, NY. No one remembers exactly when she moved, but maybe there are old address records floating around somewhere. Your maternal grandmother is Finnish and your maternal grandfather is Dutch. You once overheard your grandmother say you must have gotten your dark hair and eyes from your Italian father, etc.
A lot of the pieced together information may very well turn out to be garbage, but it’s something, and it’s a start. Just remember: it’s imperative you be ready, willing, and able to toss out the bad information and change the narrative to fit the evidence.