@Priti Upadhyay we will need for you to provide us with some information regarding your site. At this time we have no access to your site, nor any of your code. This means that we have no way of being able to identify any symptoms or causes of your trouble.
If your site is not on a public address, you might want to open your site in the Google Chrome browser, and open the Web Developer tools ( right click on any location on your page, and click 'inspect element' ) and check your 'console'. The console can report errors that you may not be aware of otherwise.
For instance you may see that you have broken links to your css files. If this was the case, then you'd want to take a second to look at how you are referencing your css files in relation to your actual file hierarchy. If you are referencing css files incorrectly in your code ( /app/skin/frontend/base/default/css/styles.css ) when your file hierarchy may be something different ( /app/skin/frontend/default/default/css/styles.css ).
Another thing to check for is that you've cleared your cache ( System->Cache Management ). This might not be the answer, but it's good to turn your cache off while developing, in case something gets cached as you're troubleshooting ( very annoying to forget this when caching doesn't reflect the current state of your code, and your code looks legit ).
Those are two things that I can think of doing. I can't really offer any more advice without a link to your site, or the code from what you're working on. It would be good to include the template file as well as the layout xml for the page you're working on.
Hope this helps!