index.php damaged? error 500

Hello everybody I hope you cn give me a hint about how to fix the error my website is givving me.
Iam running a website in Spanish and English installed in:

http://www.website.com/wp (Spanish)
http://www.website.com/wpeng (English)

Also the htaccess file has been set as:

(Spanish)
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wp/
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wp/index.php [L]
</IfModule>

# END WordPress

(English)
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wpeng/
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /wpeng/index.php [L]
</IfModule>

# END WordPress

The pther ages are working perfectly in both languages, then the problem is just with the index of both webs.

What else could be happening if everything has been working fine until today? Should I reinstall the web again? or must I run the back up generated the last time it was working fine?

Thank you very much!