Hi Mishra,
I remember, that I had the same starting problem. Of course the problem you have can have a lot of reasons. My suggestion might sound like a standard automated support mail, but it is very important, that you use the latest Code Composer Version and the latest compatible SYSbios and RTSC packages. Check your build settings also ! Get experience using the log window and the RTA tools for debugging purposes....
YES it is necessary to call BIOS_start() - not only when dealing with HWIs. BIOS_start() starts your OS - without it you won't get your system running.
Check this link for checking your setup, if you do not already know it... http://processors.wiki.ti.com/index.php/SYS/BIOS_Getting_Started_Guide