My left middle, ring, and pinky fingers are suddenly numb. What should I do?

Nerve compression. Sudden numbness indicates nerve compression. You should seek medical attention to avoid permanent nerve damage.
Change position. This is the distribution of the ulnar nerve. It runs by the elbow, and is easily traumatized from outside-"hitting the funny bone"- or inside, often by prolonged flexing like when typing on a smartphone keyboard. If numbness persists, results in muscle weakness or associated with other neurological problems, it may be a bigger problem and need workup, including EMG and imaging.