It's common for an azalea (and lots of other plants) to lose its flowers if abruptly moved from the outdoors or a greenhouse environment to the hotter, drier atmosphere of the indoors. The other, more important issue is the mixture of "regular soil" and peat in which it was repotted combined with too frequent watering. A potted azalea needs a very loose, free draining medium to grow in - lots of bark, perlite, etc. and no regular soil. Without a chance to dry out and with excess water in the soil/peat mixture, roots can easily rot away. Best thing to do is to repot into a more suitable soil-less medium and cut back the top growth which has died to healthy green tissue. Full recovery and regrowth is certainly possible, but there are no guarantees.

Thanks for your answer!

I potted it with soil mixed to peat because I read they need an acidic soil. To provide adequate draining I put a high layer of soil/peat on the bottom of the vase (which is quite tall), approx 5" or so?

I'm afraid that reporting it again will just harm it more.

5" is not the whole height, just the part on the bottom I filled before putting the plant in it.

I have some sand at hand. Could I mix that with the soil/peat to make it more loose?

Sand is unlikely to help very much and has the potential to make things worse. You need a potting mix based on peat, perlite, bark etc. such as ProMix, Sunshine Mix or other brands. The soil is the problem.

Hi Giulia,

You said, " I put a high layer of soil/peat on the bottom of the vase (which is quite tall), approx 5" or so" That was a red flag to me. Azaleas need excellent drainage. A vase is a closed vessel that holds water. Not the right container. It might be a good idea to repot with a media with good drainage in a vessel that has drainage holes in the bottom. You can always set it in a saucer as long as the drainage holes are above the water in the saucer. You may need a trivet or some marbles in the saucer to raise the container.

you are supposed to plant them in your yard, they don't make good potted plants

Nope...in cooler climates or conservatories they certainly can be grown as potted plants. The US Botanic Garden and National Gallery of Art sometimes display huge old indica azaleas grown in pots. I've seen it at Longwood too.

Thanks everyone for your comments!

I reported it using a mix of peat and a potting medium made with perlite, bark, etc (it was advertised as orchid soil).

I also put the pot on 2 block, such that the water can freely drain from the hole.

It lost basically all the flowers, but it's definitely still alive: the branches bounce back and are green, plus there are some small new leaves at the bottom.

I'll see how it does.