Two possibilities come to mind...
1) If you don't have a home.php in your theme, create one and put your error message into it. If home.php exists, it will automatically be your front page. However, if someone follows a direct link to an archive oder a post (or types in such a link as URL), your site will still be visible as it is now.
2) The next best thing would be to rename your index.php to i.e. myindex.php, create a new index.php, and then again put your error message into it.
If you have a home.php, you might consider replacing it as (2) describes for the index, and (for additional "safety") do the same with index itself.
By the time you want to revive your blog, you just (1) delete the home.php or (2) replace index.php with myindex.php