You probably deleted your /pub/static/ folder or it's contents.
Create a file called .htaccess and put these contents in it:
<IfModule mod_php5.c>
php_flag engine 0
</IfModule>
<IfModule mod_php7.c>
php_flag engine 0
</IfModule>
# To avoid situation when web server automatically adds extension to path
Options -MultiViews
<IfModule mod_rewrite.c>
RewriteEngine On
# Remove signature of the static files that is used to overcome the browser cache
RewriteRule ^version.+?/(.+)$ $1 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule .* ../static.php?resource=$0 [L]
</IfModule>
############################################
## setting MIME types
# JavaScript
AddType application/javascript js jsonp
AddType application/json json
# HTML
AddType text/html html
# CSS
AddType text/css css
# Images and icons
AddType image/x-icon ico
AddType image/gif gif
AddType image/png png
AddType image/jpeg jpg
AddType image/jpeg jpeg
# SVG
AddType image/svg+xml svg
# Fonts
AddType application/vnd.ms-fontobject eot
AddType application/x-font-ttf ttf
AddType application/x-font-otf otf
AddType application/x-font-woff woff
AddType application/font-woff2 woff2
# Flash
AddType application/x-shockwave-flash swf
# Archives and exports
AddType application/zip gzip
AddType application/x-gzip gz gzip
AddType application/x-bzip2 bz2
AddType text/csv csv
AddType application/xml xml
<IfModule mod_headers.c>
<FilesMatch .*\.(ico|jpg|jpeg|png|gif|svg|js|css|swf|eot|ttf|otf|woff|woff2)$>
Header append Cache-Control public
</FilesMatch>
<FilesMatch .*\.(zip|gz|gzip|bz2|csv|xml)$>
Header append Cache-Control no-store
</FilesMatch>
</IfModule>
<IfModule mod_expires.c>
############################################
## Add default Expires header
## http://developer.yahoo.com/performance/rules.html#expires
ExpiresActive On
# Data
<FilesMatch \.(zip|gz|gzip|bz2|csv|xml)$>
ExpiresDefault "access plus 0 seconds"
</FilesMatch>
ExpiresByType text/xml "access plus 0 seconds"
ExpiresByType text/csv "access plus 0 seconds"
ExpiresByType application/json "access plus 0 seconds"
ExpiresByType application/zip "access plus 0 seconds"
ExpiresByType application/x-gzip "access plus 0 seconds"
ExpiresByType application/x-bzip2 "access plus 0 seconds"
# CSS, JavaScript, html
<FilesMatch \.(css|js|html)$>
ExpiresDefault "access plus 1 year"
</FilesMatch>
ExpiresByType text/css "access plus 1 year"
ExpiresByType text/html "access plus 1 year"
ExpiresByType application/javascript "access plus 1 year"
# Favicon, images, flash
<FilesMatch \.(ico|gif|png|jpg|jpeg|swf|svg)$>
ExpiresDefault "access plus 1 year"
</FilesMatch>
ExpiresByType image/gif "access plus 1 year"
ExpiresByType image/png "access plus 1 year"
ExpiresByType image/jpg "access plus 1 year"
ExpiresByType image/jpeg "access plus 1 year"
ExpiresByType image/svg+xml "access plus 1 year"
# Fonts
<FilesMatch \.(eot|ttf|otf|svg|woff|woff2)$>
ExpiresDefault "access plus 1 year"
</FilesMatch>
ExpiresByType application/vnd.ms-fontobject "access plus 1 year"
ExpiresByType application/x-font-ttf "access plus 1 year"
ExpiresByType application/x-font-otf "access plus 1 year"
ExpiresByType application/x-font-woff "access plus 1 year"
ExpiresByType application/font-woff2 "access plus 1 year"
</IfModule>
and put it in the /pub/static/ folder.
Also clear static files and cache just to be sure, by running:
php bin/magento setup:static-content:deploy
php bin/magento cache:flush
These 2 commands are the only thing you need to run if you made changes to css/js/phtml files.