Well I can clearly see in the Haartraining documentation that if it is interrupted, it has to be started from stage 0 again. However, by mistake when I re-started my HaarTraining, I forgot to change the data folder name, and the training actually got started from stage 20 (the previous data folder already had training data upto 19 stages).
Now the question is, if I continue that training would the classifier be usable or I am wasting time and should restart training with new data folder with stage 0.