Looking in the stack trace the error is occurring is here:
app/code/core/Mage/Core/Model/App.php(491): Mage_Core_Model_App->getStore();
Can you give us more background on this? Did your site ever work? Have you recently changed the store settings eg deleted a store?
If you use a different browser or a different computer to access your website does that work?
Cookies are used to tell Magento which store you want to visit so try removing all your cookies.
What else? Maybe an XML file is missing or erroneous and the store setting can't be found by Magento configuration.
Try some of these things then please give more background: eg Is this a fresh Magento install? Can you access the Magento admin pages? Has this ever worked? Are you in production? Deve;opmet? Running locally on XAMP? Recently changed your store configuration? Recently changed or moved XML files? Switched to a new theme?