I deleted /devel_themer folder now my website reports an error devel_themer folder of my website that was in public_html had about 500MB size so I deleted this folder. now when I log in to administrative account it reports this error :

Fatal error: Maximum execution time of 30 seconds exceeded in /home/my_web_site/public_html/sites/all/modules/devel_themer/devel_themer.module on line 342

now what shall I do ?
I know its answered in comment, but for others who land here.

Increasing the execution time in php.ini can solve this. An ideal value would be 120. 

If it doesn't solve the problem we might need to disable and uninstall the module and fix the problem. Ofcourse you can install it back after things are fixed.

If you can access the admin page:


Disable the module.
Uninstall the module from the uninstall tab in modules page and delete the module folder.
Clear the site cache if required.


If you can't access the admin page:


Delete the module folder. 
Run update.php


If you were not logged in at the time of the error you will have problem accessing the update.php, then you will have to either use drush the command drush updatedb or temporarily enable access to update.php by changing settings.php file.

If you had drush, you an also consider disabling the module with it.

Line to change:

$update_free_access = FALSE;