Hi Murli,
Are you getting this issue while viewing CRM form or everywhere. if you are getting this issue only on CRM form then I think some JavaScript error is causing this issue. Can you please provide the screen shot?
Please refer these blogs.
http://thinketg.com/dynamics-crm-2011-ru-12-blank-ribbon-issue-resolution/
http://microxrm.blogspot.in/2013/01/crm-2011-ribbon-not-loading-in-mozilla.html
Thanks!
Kalim Khan