What happens if blood transfusion fails in the second stage of blood cancer?

the person has already taken 7 times blood transfusion..and its been successful all 7 times..

There generally is no question about whether blood transfusions will 'fail'. The question is whether or not the patient will have a reaction to the blood. You can react to the perservatives in the blod, as well as the antibodies in the blood. The person will know they are having a reaction by breaking out in hives, rapid heart rate, short of breathe, pain, ect.
Most people are given premeds before a blood transfusion, especially if they have reacted in the past. Premeds are usually tylonal and benedryal.

If the person does react, the transfusion is stopped, meds are given, and then another unit is started a few hours later.

I guess ultimatly if a transfusion were to 'fail', they would simply try another unit. A lot of times a cbc is taken about an hour after transfusion to see how much the unit bumped up the numbers. If the patient was unable to receive blood... It could be quite detrimental. If it is actual blood, well, the person could suffocate to death.. blood is given when the hemoglobin levels are too low. Without hemoglobin, you cant breathe and your heart brain and rest of the body arent getting oxygen. But, assuming the patient is just going through treatment, their own marrow would kick in eventually and start reproducing blood cells. If this is a transplant patient, and the transplant failed, and they couldnt receive transfusions, they would die.

And a person with a leukemia shoul expect to recieve a lot of blood transfusions. While I was going through treatment I had to receive platelets twice a week, and blood once a week. I cant even begin to tell you how many transfusions I have had total.