An update: I charged the battery for the entire night and got a 100% charged indication in the morning. But when I disconnected the charger and tried to turn on the phone, I have the same issue. I still get the yellow triangle with the exclamation mark.
But this time around, I was able to enter the recovery mode by pressing the Volup+VolDwn+power key and I rebooted the phone from the recovery screen and it worked fine. I am able to run the downloaded apps, make calls etc.
Now, I notice 2 new things:
1) Every time I turn the phone off or when it turns off automatically (which it does very frequently since today), it doesnt restart normally. I have to enter the recovery mode to reboot the phone and make it operational
2) When the phone is turned on and when I charge it, no matter how long I charge, the charge percentage shows 78% always.I guess the only time I get a full 100% charge indication is when the phone is turned off and then charged.
All this is leading me to believe that there might be a problem in the battery. If the phone operates properly from the recovery mode, I guess there is no issue with the mother board. Any ideas?? Please suggest.