Please help with Identifying the diode

SMD devices typically are marked with random letters/numbers, but through-hole devices usually try to use markings that resemble the actual name.

"V21" sounds like the BAV21, which is a general-purpose diode made by many manufacturers.

This failure is the tip of the iceberg. TVS stands for transient voltage suppressor. It's job was probably to clip unusually high voltages to keep the rest of the circuitry safe.

If the above is true, then sufficiently high voltage at sufficiently long duration and low impedance came along to blow out the TVS. This means it no longer served its protective function, so the high voltage most likely proceeded to blow out other components. Very likely, there are several dead components now on that board.

Basically, that board is toast. Unless you have a schematic or a service manual, there is little you can do about it except replace it.

However, before replacing the blown board with a new one, try to figure out what killed the first board. Did you have a nearby lightning strike? Is the unit intended for 120 V and was somehow plugged into 240 V? If you don't identify the problem, you can't know the new board won't be subjected to the same over voltage, and fry as soon as you install it.