What may happen if mantra chanting is stopped for few days because of busy life schedule?
Will the punya or power earned by chanting it will be lost if chanted irregularly?
There are several different aspects to this question. First, consider what happens to you if you do not wash one day, or do brush your teeth. Your body will not die nor will your teeth fall out, but you do feel a little dirtier - you are a little more immersed in the world. Likewise with your japam. If you do not do japam for one day or a few days, your become a little more immersed in the world.
You do not lose the japam that you have already done; as Krishna says (Gita VI. 40):
O Partha, there is no destruction for him either in this world or the next; no evil, My son, befalls a man who does good.
Likewise, you do accumulate any bad karma for not doing Japam. Sankara says in his commentary to verse IV. 18 in the Gita (Swami Gambhirananda translation):
Nor even can any evil, which is an entity, arise from the non-performance of nityakarmas [daily obligatory duties] which is a non-entity, for there is the statement, 'Of the unreal there is no being' (Gita 2.16), and (in the Upanishad) it has been pointed out 'How can existence originate from non-existence?' (Chhandogya U. 4.2.2). Since emergence of the existent from the non-existent has been denied, therefore anyone's assertion that the existent originates from the non-existent will amount to saying that a non-entity becomes an entity, and an entity becomes a non-entity! And that is not rational because it runs counter to all the means of valid knowledge.
Further, the scriptures cannot enjoin fruitless actions, they being naturally painful; and it is illogical that what is painful should be done intentionally. Also, if it is admitted that falling into hell results from their non-performance (i.e. of the nityakarmas), then that too is surely a source of evil. In either case, whether one undertakes them or not, the scriptures will be imagined to be useless. And there will be a contradiction with your own standpoint when, after holding that the nityakarmas are fruitless, you assert that they lead to Liberation.
Having said all that, what then will happen if you do not do for a few days? Like not walking or exercising for a few days, you will gradually use the strength that your mind has built up in doing the austerity on a daily basis. Your mind will get a little lazy and convince you that a few days is not so bad - you can build up your strength again.
What to do? You need not sit and do the Japam, can you not do it while walking, or bathing, or eating? Put part of your mind while doing your other daily activities and you can still do your daily Japam. I am sure you still find time for eating and bathing! Do Japam while doing other things!
There is a Zen Buddhist saying: Meditate for 15 minutes every day. On days that you do not have time to meditate for 15 minutes, meditate for 1 hour.