ecommerce blank page

Every single time I install any ecommerce plugin, it doesn’t fail but my website just shows a blank page if I type in the domain name. Once I deactivate my website works again, but I’ve used multiple plugins and get the same problem. I think it might be my .htaccess, but I already increased the memory limit to 128M. What else could it be? Here’s my .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^files/(.+) wp-includes/ms-files.php?file=$1 [L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule . index.php [L]
</IfModule>

# END WordPress

php_value memory_limit 128M