This webpage is not available

first, check if the server is down. have you contacted your hosting provider about this?

This seems to be a server issue, not WP core issue. So, first, make sure the server is not down

Usually a hosting provider has more one server. The other site although may be with the same host, but it could be hosted ona different server.

Also, there may be some other problem with the account. If there is, they are the only one who can explain.

But, your issue is certainly not WP core issue.

I can list here a bunch of things to try, but that may further mess up things.

try a few basic troubleshooting steps:
– Flushing any caching plugins you might be running, as well as server and/or browser caches.
– deactivating ALL plugins temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s). When you can’t get into your admin dashboard, try deactivating via FTP or SFTP or whatever file management application your host provides. Also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old.
– If that does not resolve the issue, try switching to the unedited WP default theme (such as Twenty Fourteen Theme etc.) for a moment using the WP dashboard to rule out any theme-specific issue. When you don’t have access to your admin area, you can switch to the default theme by renaming your current theme’s folder in wp-content/themes and adding “-old” to the end of the folder name using via FTP or SFTP or whatever file management application your host provides. Alternately, you can remove other themes except the default theme. That will force your site to use it.
– If that does not resolve the issue, try resetting the plugins foldervia FTP or SFTP or whatever file management application your host provides. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).
– If all the above steps do not resolve the issue, then try MANUALLY re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones. Read the Manual Update directions first!
– running the upgrade manually via wp-admin/upgrade.php
– BTW, if you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong. It’s a great safety net.