HI
If in case you have lost the Muse file or the computer is crashed due to which you do not have the access to file,I am afraid, at this stage its not possible to import the site that you have published back in Adobe Muse
Also, to be on safer side, you may keep your file save with creative cloud storage you get when you purchase your subscription so that, even if you don"t have access to you computer, you may have the updated file downloaded to another and do not loose the work.
Let me know if you have any further query.