Why my phone is not charging?

I was browsing Internet, then suddenly my phone start to heat & it was increasing. Then I turn it off & after a few time connect it to charger in off mode. I saw in display that remaining 34% of charge. After 2 hours when I came to unplugged the charger I was shocked to saw that it still showing Battery 34%. Suddenly the percentage start to decreasing although it was connecting to charger. After a short time the battery percentage gone to 0% & the phone completely shut down. Now it's not charging or getting on? What can I do Now & what is the main reason behind it? Is tha a major problem? How can I fix my mobile phone as before?