You can download Muse from here, http://www.adobe.com/go/trymuse/
After it is installed you can sign in using your Adobe ID, if you have an active subscription on the ID then it should recognize it.
I'm really sorry to hear that your computer has crashed. However, I have some bad news. Unless you had it stored in the Creative Cloud or somewhere else, there really isn't a way to get the .muse file back. You can retreive your html/css information if you published it to a web server, however, you won't be able to make changes in Adobe Muse.
You really only have two options: A) Start from scratch and recreate your site in Muse, B) Manually update your site through code if your site is hosted on a web server.
Hope this helps!
Europe, Middle East and Africa