This often happens for two reasons,

1. You removed/added/changed a module without disabling the configuration cache first
2. Configuration cache corruption

To solve 1.

Clear your configuration cache and if the issue is resolved, nothing further is required.

To Solve 2.

This is a core-code bug in some versions of Magento. You can test if it is caused by this bug by editing Mage_Core_Model_Config to reflect the following.

public function init($options=array())
{
    $this->setCacheChecksum(null);
    $this->_cacheLoadedSections = array();
    $this->setOptions($options);
    $this->loadBase();

    $cacheLoad = $this->loadModulesCache();
    if ($cacheLoad) {
        return $this;
    }
+   $this->_useCache = false;

    $this->loadModules();
    $this->loadDb();
    $this->saveCache();
    return $this;
}