Could you provide some more information please. What is your PHP version? Does it work with a default starter kit? What is in your config.php (if you have more than one, post all), what is in your .htaccess.
Sure there were no changes made on the server side (maybe through hosting service)?
# Kirby .htaccess
# rewrite rules
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
RewriteEngine on
RewriteCond %{REQUEST_URI} /+[^\.]+$
RewriteRule ^(.+[^/])$ %{REQUEST_URI}/ [R=301,L]
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault "access plus 1 month"
ExpiresByType image/x-icon "access plus 1 year"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType text/css "access plus 1 year"
ExpiresByType application/javascript "access plus 1 year"
</IfModule>
<IfModule mod_headers.c>
Header always set X-Content-Type-Options "nosniff"
</IfModule>
<IfModule mod_headers.c>
Header always set X-XSS-Protection "1; mode=block"
</IfModule>
<IfModule mod_headers.c>
Header always set X-FRAME-OPTIONS "DENY"
</IfModule>
# enable awesome urls. i.e.:
# http://yourdomain.com/about-us/team
RewriteEngine on
# make sure to set the RewriteBase correctly
# if you are running the site in a subfolder.
# Otherwise links or the entire site will break.
#
# If your homepage is http://yourdomain.com/mysite
# Set the RewriteBase to:
#
# RewriteBase /mysite
# In some enviroments it's necessary to
# set the RewriteBase to:
#
# RewriteBase /
# block text files in the content folder from being accessed directly
RewriteRule ^content/(.*)\.(txt|md|mdown)$ index.php [L]
# block all files in the site folder from being accessed directly
# except for requests to plugin assets files
#RewriteRule ^assets/plugins/([a-zA-Z0-9\.\-_%=]+)/(.*)$ site/plugins/$1/assets/$2 [L,N]
#RewriteCond $1 !^plugins/[a-zA-Z0-9\.\-_%=]+/assets/.*
RewriteRule ^site/(.*) index.php [L]
# block direct access to kirby and the panel sources
RewriteRule ^(kirby|panel\/app|panel\/tests)/(.*) index.php [L]
# make panel links work
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^panel/(.*) panel/index.php [L]
# make site links work
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*) index.php [L]
</IfModule>
# Additional recommended values
# Remove comments for those you want to use.
#
# AddDefaultCharset UTF-8
#
# php_flag short_open_tag on
Apart from that, I’d clean this up a bit, so a not to have the headers and expires inside the mod_rewrite:
# Kirby .htaccess
# rewrite rules
<IfModule mod_rewrite.c>
RewriteEngine On
#RewriteCond %{HTTPS} !=on
#RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
# enable awesome urls. i.e.:
# http://yourdomain.com/about-us/team
RewriteEngine on
# make sure to set the RewriteBase correctly
# if you are running the site in a subfolder.
# Otherwise links or the entire site will break.
#
# If your homepage is http://yourdomain.com/mysite
# Set the RewriteBase to:
#
# RewriteBase /mysite
# In some enviroments it's necessary to
# set the RewriteBase to:
#
# RewriteBase /
# block text files in the content folder from being accessed directly
RewriteRule ^content/(.*)\.(txt|md|mdown)$ index.php [L]
# block all files in the site folder from being accessed directly
# except for requests to plugin assets files
#RewriteRule ^assets/plugins/([a-zA-Z0-9\.\-_%=]+)/(.*)$ site/plugins/$1/assets/$2 [L,N]
#RewriteCond $1 !^plugins/[a-zA-Z0-9\.\-_%=]+/assets/.*
RewriteRule ^site/(.*) index.php [L]
# block direct access to kirby and the panel sources
RewriteRule ^(kirby|panel\/app|panel\/tests)/(.*) index.php [L]
# make panel links work
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^panel/(.*) panel/index.php [L]
# make site links work
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*) index.php [L]
</IfModule>
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault "access plus 1 month"
ExpiresByType image/x-icon "access plus 1 year"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType text/css "access plus 1 year"
ExpiresByType application/javascript "access plus 1 year"
</IfModule>
<IfModule mod_headers.c>
Header always set X-Content-Type-Options "nosniff"
</IfModule>
<IfModule mod_headers.c>
Header always set X-XSS-Protection "1; mode=block"
</IfModule>
<IfModule mod_headers.c>
Header always set X-FRAME-OPTIONS "DENY"
</IfModule>
# Additional recommended values
# Remove comments for those you want to use.
#
# AddDefaultCharset UTF-8
#
# php_flag short_open_tag on