Yesterday, I had power failure and my Pc turned off and when I next started firefox, there was nothing in my bookmarks.
I lost all my bookmarks.

Can any one help how I can get them back ?

its easy..

Actually Firefox saves daily 5 backup of bookmarks, so you need to import any of recent backup,
First Goto
Bookmarks > Organize Bookmarks > File > Import... > From File

Now you have to navigate to bookmark backup file.. which can be found at following place.
Goto C:\Documents & settings > (Then your profile in Windows XP) > Application Data >Mozilla > Firefox > Profiles > (Now select your profile which you are using in Firefox) > Bookmarkbackups

Now there you will find bookmark backup file and done !

Note : Make Sure you have set Hidden file to be visible.