In Spain bought an orange tree this year which had lots of flowers on it. These have all fell off and the tree now looks limp. I cannot see any bugs and have watered it well.
Blossom drop is not unusual for any citrus or other fruit tree. That is discussed in this article: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-blooming-season.htm
You state the tree has been well watered. Is it in a container? Is the water running right through the soil...really fast? If so, it's very likely that at some point the soilball has become too dry. To rehydrate, simply soak the entire soilball in a large bucket or tub overnight.