Hello
(Hint: We Swiss like to be greeted, we consider that polite.)
That is generally hard to answer, as it depends on quite a few variables.
It is possible that you booked your ticket on a train which has to be rerouted or partially cancelled due to construction works. There are construction works between Feldkirch and Buchs, but that is only one possible way to travel between Switzerland and Vienna.
Whether your ticket is valid on all trains or only a specific connection depends on the type of ticket. For example, Swiss Supersaver’s tickets (German ’Sparbillete) are limited to one connection, while regular point-to-point tickets ('Streckenbillete) are not.
In Austria, there are Sparschiene (roughly equivalent to the Swiss Supersaver), Semi-Flex (also bound to a connection) and Flex (any connection allowed). In the case that you have a ticket restricted to a single connection, the ticket vendor should already have contacted you and offered you an alternative.
It is really hard to see which cases apply to you as unfortunately, my cristall sphere is a bit cloudy today… Please provide the type of ticket and where you bought it and the travel date and route.