This is a common Wordpress error which can be occur anytime. You can follow below steps to resolve this error:
- First check your .htaccess file or try it after renaming the file.
- You can increase the memory limit and then try to access your website.
- Deactivate all the plugins and then check your website
- May be any core files are missing, so upload core files again.
- If any of these methods doesn't work then it is must to contact your hosting provider.