So it could be that the train shows as cancelled and is effectively cancelled stricto sensu but that a service with the same number at the same time will run using a replacement frame. This is one peculiarity of DB: if a the train goes tech before departure, they put a replacement train at the same time and with the same number, will notify you the train is "cancelled" but will not tell you about the replacement one in the app. The information board will show the train as running properly. So check the TV. Utterly bizarre, this has happen to me several times. In that case just go on the new train, take your assigned seat if any and show your old ticket at control.
Another scenario is that it is truly cancel, information board shows as cancel. If you have a Flexipreis ticket, then you can take any other train (you lose your seat reservation if any). If you do not have a Flexipreis, you need to buy a new ticket for a running train, go to DB Reisecenter and ask for a delay form, fill it to get refunded the ticket of your cancelled train.