Have you been using Yahoo! Site Explorer for a few years? Sad news, the free backlink service is now finally completely shut down. It might have been a few months later than first announced, but now it’s totally gone.
Yahoo recommends using Bing Webmaster Tools instead. Bing Webmaster tools can be excellent in helping to determine inbound links, deep links and much more related to search engine optimisation. But it is only possible to analyse your own inbound links and cannot be used to analyse competitors.
For the purpose of finding backlinks to competitors you now have to go with Open Site Explorer by SEO Moz or similar link measuring solutions.