Most ordinary HO uses the Horn Hook coupler. The problem is in the adapter or mount used.The local Hobby shop will have couplers but the adapter may needed from a junker to scavenge. It depends what broke. This of course I am speaking generally without specifics.
The first picture shows a stanard coupler.The second shows one broken off and the box cover used to keep it in place.
If it is like my old entry level Bachmann and has the coupler on the truck then you can replace the horn hook itself. If the piece that hold the coupler to the engine is broke then you will need to replace the powered truck. It is cheeper to buy a parts engine on EBay in this case.
I had to order a coupler mount for a Bachmann Spectrum engine from Bachmann's web site. The have exploded views of some of the engines. If you can find the part it will be cheep. Unfortunately shipping was about $8 or $9.