to many reason . many be user try to come on your blog directly
and the face page not found error,or missed any word from your blog url. second reason is may be you had deleted your post so when user come from search engine get 404 error page (page not found) message.
Once google search engine index your blog post, search engine start crawling that post.When some one search query related to that post, google show your post in search engine result but if visitor view your post blogger show 404 error.This error is due to deleted post and if you moved that link to other.You can recover by editing redirect.
redirect 404 error in blogger
step1:Go to blogger dashboard and then
go to setting > search preference
step2: now > custom redirects , edit it
Step3 paste the below code there
Sorry, the page you're looking for in this blog does not exist.
You will be redirected to homepage shortly.
<script type = "text/javascript">
BSPNF_redirect = setTimeout(function() {
location.pathname= "/"
}, 5000);
</script>
and / with your page url.
now save it and you have done it.Your page not found error fixed.
If you do not fix 404 error then you may lost lot of visitors.Blogger give manually option to fix this error.After setting redirect link, user go to 404 not found page will redirect to home page.