During a schedule change or aircraft type change, everyone tends to temporarily lose all seat assignments. Sometimes you can reassign yourself seats during the process, but they might not stick.

A search doesn't show flights for which tickets are no longer being sold, so if your flight is really overbooked to the maximum permitted, it can look like the flight no longer exists when in fact it has not been cancelled.

Have you tried calling DL to ask whether there has been a schedule change and to attempt to verify your seats?