I'm not affiliated with Xanga, but in order to keep blogging, it looks like you have to:
1) Download your blog archive (a .zip of .xml files) from the Dashboard. There should be a yellow bar with a blog archive link at the top of the page. This may not be done for your blog yet, but they appear to actively be working on this for everyone (whom logged in in the last 5 years) for free.
2) Pay for a subscription to Xanga 2.0, have financially supported Xanga 2.0 relaunch, or have paid for Xanga Premium.
3) Find the Tools link on the Dashboard page and import your blog archive

"Migration" is a bit of a vague term. For people that are willing to pay for a subscription, migration means your blog is moved to the new system and re-deployed as a blog that you can see, edit, and continue posting on. For those not having paid, it merely means you will be able to download a raw-data archive of your posts from a blank Dashboard. Hopefully, you can import them at some other provider. If you, like me, have never given Xanga a dime, and aren't going to now, then at least you have access to your blog posts saved in the blog archive.

Here's more info:
- http://help.xanga.com/topic/please-read-if-your-site-isnt-showing-up
- http://thexangateam.xanga.com/2013/09/05/were-working-to-make-2-million-blog-archive-backups-available-for-free-download-asap/
- http://thexangateam.xanga.com/2013/05/30/relaunching-xanga-a-fundraiser/

Your blog will not be available to be hosted on Xanga unless you pay a subscription fee. If you wish to remain with Xanga, and have not purchased a subscription you can do so here: https://xanga.crowdhoster.com/relaunch-xanga

If you do not wish to pay the yearly subscription and wish to move to another blog hosting service -- the process is fairly simple. Your blog archive is in a WXR format. Wordpress.com makes the transition simple in that regard. If you have made an account on Wordpress.com follow these instructions. http://en.support.wordpress.com/import/
Other hosting services may have different requirements, and you may need to look for conversion tools or copy/paste the entries manually.