Here's what I know so far. The AC adapter is fine, the DC jack in laptop is fine. The battery is fine as well.
The issue I am facing is that the battery would stop charging in between and the system would keep switching back and forth from DC to AC and then DC again and then would finally turn off when the battery would drain to 0. So basically the real issue is that the system would simply not run on AC supply. Now this all started after I applied thermal paste to CPU and GPU. I have given my laptop for repair and the guy is unable to find the issue. He just says that something is shorting in the motherboard. Hence I am posting here for any kind of help if you would know by your experience the cause of this problem. Any kind of suggestions, guesses, help would be welcomed. Thanks.

 

You may get better help if you give more details, eg. the make and model of laptop.
Can you run the laptop off the AC adapter alone, after removing the battery?
How have you determined that the adapter and battery are "fine"? Have you tried both with a similar machine?
These kind of things can be difficult to diagnose and even harder to fix. I had a similar problem and literally searched for and bought a working model of the same laptop. Once I was done, I sold it again and lost nothing on it.