This situation reminds me a lot of the one I was in, last year, in October, riding ...the Railjet (high speed train) from Vienna to Munich.
(your exact route)
Let's just say: the train I booked, with reserved seats, went....without us. I got email notices from OeBB (not DB in this case) but they turned out to be false. (that is, there was no problem, when they notified me twice by email, there were delays).
In that case: I did spend time patiently waiting at the travel reservation office (the one with the "take a number", wait for 100 people ahead of me while I nearly fainted from stress). And the representative just printed out all the options and connections for later trains. And told me to get on the next one.
The downside: the 4 hours to Munich (which extended to longer), because my seat reservations were gone, were miserable with constantly roaming the overpacked train, looking for a seat to sit in, having to stand between dirty bicycles and luggage, and not step on people crouched on the floor in the train. Which is not nice for my senior parents, who aren't really suited to having stand up or roam around or keep getting kicked out of their seat because the train came to a stop where there was a passenger getting on with a reservation. And trying to not fall out the door when there are so many passengers I am pressed up against the end of the car (with other passengers, and their luggage).
And when I got to the DB train connection, and changed trains at that station, the DB train sat there and was delayed 45 minutes. For no reason. (The German passengers asked the platform conductor and also got no reasons). But OK, I didn't care, because I could finally sit down.
So after the "trauma" of this:
(this is for the RailJet ticket...and you're on a 1st class)
a) you can get on another train
b) it could be crowded if your train got cancelled because everyone is getting on other trains who were supposed to get on the one you booked. If you can get reservations (seats) on another train, that might be helpful (? is that an option if your ticket is for a different time?) .
c) have patience. You're going on a OeBB train to a DB destination. Maybe...another notice will come through and the cancellation would be reserved, and the trip will be back "on " again? But be prepared that arrival in Munich could take longer than originally planned. (e.g. do not plan some tight scheduled activity that you have to arrive in Munich by a certain h:mm).
I do not have any illusions about "how wonderful high speed rail was from Vienna to Munich" is anymore. But it beats having to use airlines. I just adjusted my expectations.