Fatal error: Class 'Varien_Data_Collection_Db' not found in /home/xxx/public_html/app/code/core/Mage/Core/Model/Resource/Db/Collection/Abstract.php on line 36
Without more details it is difficult to debug,Store not working after successful SUPEE-6788_CE_184.108.40.206 patch
As @muk_t said, more information is required in order to help you debug this issue.
Good documentation of the issue your experiencing is half of the solution. Without that, there are many scenarios that could be leading to your error and it's difficult to help you find the another half of the solution.