The leaves on my mini rose(house) plant have started turning yellow and falling off. What should I do?

Make sure you don't have any spidermites or, that it's been over or under-watered. Spidermites love those dry conditions. I found mine grew better in bright light with a little direct sun rather than full on hot sun indoors, and I never let it dry out completely, soil should feel on the semi moist side and they also like some humidity. If you have a large clear vase of some sorts, setting it down in that with just a little water UNDER some pebbles would help, or even a mini greenhouse that Big Lots have sometimes.
Because they are roses, they do so much better outside with half a days sun planted in the ground then indoors.