Getting constantly internal server error 500 my host changed it twice but it happen again

What WP version are you running? And which theme? PHP version? Troubleshoot as follows, one by one and see what is the result: —
1. Activate the stock theme, not customized theme.
2. Deactivate all plugins.
3. reset plugin folder.
4. Check your database, repair any tables with overhead, and optimize.
5. Back up database.
6. Update WP. Or if WP version is latest, update core.
7. Put website in maintenance mode and delete all folders and files EXCEPT wp-content, wp-config and .htaccess. Download fresh WP install, and upload the new folders and files, all EXCEPT wp-content, wp-config and .htaccess.

Does the server 500 error happen only when you try to change your WP theme? Or is it possible that excessive server activity and demand for memory is causing server error in your shared hosting account? If this is the case, it might not be related to your WP installation; rather it might be due to brute force attacks on your wp-login.

As for you bruter force attack I have installed wpfence security secondly what else i can use to help, limit login or wp lockdown but they cause problem for me sometime, the defualt username is not what shows on the website , I am on 3.8 version all plugin and themes only two theme are there graphene and mystic 2, so the defualt theme is mystic in my current case, as for memory use my cpanel shows i amusing only 16 percent, I had backed up data bas, yes this happed whenever i tried to install update new post or do anything on panel I am thinking of reinstalling the whole thing how about the idea

today i tried to install a new theme nothing happened but when i added a new post got 500 internal server error , the server admin had modified the htaccess so many time but what is the problem cant figure it out

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

server error log shows
[Sun Jan 19 06:46:25 2014] [alert] [client 46.229.164.98] /home/netalien.biz/public_html/.htaccess: </IfModule> without matching <IfModule> section

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress
I used only this in a htaccess file which was empty there is also two htacces files i can see better wp security and total cache having the same code in the starting and end of the htaccess.bk folder should i add something else here

the site still show internal server error what else is requierd

currently it shows

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 7680 bytes) in /home/netalzdx/public_html/wp-includes/wp-db.php on line 1031

while opening the site