Hello and welcome to the forum. I am sorry to hear about your uncle.
The possible causes of coma after heart surgery are extensive and can include things that are very serious but also things that are much less serious.
For instance, he could have suffered a stroke which would be quite serious. Alternatively, he could simply be suffering from the after effects of anesthesia and cardiac surgery which would be generally transiet and should improve.
Unfortunately, I dont think I can provide you much further insight. I encourage you to discuss the details with your uncle's physicians.
Wishing you all the best.