Visitors browsing through the YouTube channel of the pianist Valentina Lisitsa can watch her in hundreds of videos. There are live webcams of her practicing at her home in North Carolina, long blonde hair tossing and brow furrowed in concentration as she reads through new works. There she is in a red gown playing Schumann’s “Traumerei” at a concert in Seoul, and recording Rachmaninoff’s Concerto No. 3 at the Abbey Road Studios in London. Ms. Lisitsa, 43, resurrected a completely stalled career through YouTube.