I think that your back links ahs been removed by the owner of the webmaster where you have build them.
thanks a lot! i have checked some of those sites and found that they are all ok and my links are still threre~~
I guess the site that give you backlinks has been PR downgraded, or penalized by google, therefore it's no longer a quality backlinks.
Don't worry,it's not your fault.The websites which had back links to your site has probably deleted them.
first try to find how many and frm where you have lost the links......check through those websites by keeping in mind that whether throug any ways you have not abided their privacy policy or violated any T&C
hi you should start all activities of seo process like directory sub, article sub, link building, blog and forum participation.