If you used the Del.icio.us bookmarking website and accessed it via your Yahoo! account, you might well have lost access when the website was transferred to its new owners (i.e. if you didn't opt in to have your account transferred across).
However, it seems that the original servers are still accessible, albeit not through the same domain name.
The trick, therefore, is to edit your hosts file to visit the old servers through their old URLs. I got the instructions from a comment on this Yahoo! Answers post, which in turn got it from a comment on this GigaOM post.
Effectively, the answer is to edit your hosts file, adding the following:
You can then log in to your old Delicious accounts and export your bookmarks. Then you can import them wherever you like. I chose to go with Pinboard, which has a very reasonable $10 one-off fee and seems to do everything I want.