If you extract Blesta and move the files sometimes the htaccess doesn't move over or if you've accidentally mucked up editing it, this is the default raw htaccess..

########################################################
# package:    minPHP
# filename:   .htaccess
########################################################

# Do not allow direct access to templates
# or an unauthorized backdoor from CE
<Files ~ "blesta2ce.php|\.(pdt)$">
   order deny,allow
   deny from all
</Files>

# Protect against Clickjacking
#Header append X-Frame-Options "SAMEORIGIN"

RewriteEngine on

# Disable Apache MultiViews since it
# may conflict with the GUI installer
<IfModule mod_negotiation.c>
    Options -MultiViews
</IfModule>

# Force HTTPS
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [R=307,NE,L]

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ index.php

RewriteCond %{REQUEST_URI} ^(.*)/install.php$
RewriteRule install.php %1/install/ [R=301,L]