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.