After installing Devel Module (and Library simplehtmldom), the Drupal7 site is completely dead. Have already set the module to 0 on System table in database, and the module files removed by FTP. But still no recovering. How to remove simplehtmldom Library? What else to recover the site? Thanks.
You can try using Registry Rebuild
Project page says..
THIS IS NOT A MODULE. YOU CAN'T ENABLE IT. FOLLOW THE INSTRUCTIONS.
Make a backup of your database before running this. It won't break your database, but we can't improve this unless we have broken databases that it doesn't quite do the job for.
There are times in Drupal 7 when the registry gets hopelessly hosed and you need to rebuild the registry (a list of PHP classes and the files they go with). Sometimes, though, you can't do this regular cache-clear activity because some class is required when the system is trying to bootstrap.
When would you need Registry Rebuild? You might get something like:
PHP Fatal error: Class 'EntityAPIControllerExportable' not found in ...sites/all/modules/rules/includes/rules.core.inc on line 11
Look in the error log to see an actual error. Put the modules you deleted back in the folder. Try uninstalling them with Drush.
Also, Drush can help you diagnoze - it can print error messages even if the whole site goes WSOD.
Basically, devel + htmldom won't ruin your site, unless you are tight on memory. If that's try, set max memory in php.ini to 256M or 512M.
And on the contrary, you can try deleting these modules folder manually and seeing of the site runs. Also, you can try to remove all the contrib modules folder as a whole and seeing if that helps.
Some more things - check if error messages are disabled (should be enabled) for development in php.ini. I fyou have little memory and no error messages enabled, that any case like htis will be a nightmare ;)