One of my coin is now in great loss.How can I overcome this? Should I sell them all??Or I have to wait more.?This may be the great loss I ever face??How I can be more careful about buying coin in future?Please,help me out..
There isn't really any simple answer to your question. The only real way to overcome the loss from the particular coin that you mention is to hold it and hope for the best but that really depends on what the coin is and the reason for the drop.
If your coin has lost value due to the recent spike in ETH and BTC your best bet is to wait until the market corrects itself, which it will.
AS for your future investments I don't really have much advice other do your research before investing. Is the company respected? Does it solve and actual problem? Are you buying during a spike? Do they have any plans for the foreseeable future? What does their roadmap indicate regarding their plans? Have you read their white paper?