Magento Issues need MASTER MAGENTO EXPERT

We have some issues with our Magento. We cannot login to it and there is an error Warning: simplexml_load_string() [function.simplexml-load-string]: Entity: line 54: parser error : StartTag: invalid element name in /home/domain/public_html/lib/Varien/Simplexml/[login to view URL] on line 383 #0 [internal function]: mageCoreErrorHandler(2, 'simplexml_load_...', '/home/domain...', 383, Array) #1 /home/domain/public_html/lib/Varien/Simplexml/[login to view URL](383): simplexml_load_string('loadCache() #3 /home/domain/public_html/app/code/core/Mage/Core/Model/[login to view URL](408): Mage_Core_Model_Config->loadModulesCache() #4 /home/domain/public_html/app/code/core/Mage/Core/Model/[login to view URL](338): Mage_Core_Model_App->_initModules() #5 /home/domain/public_html/app/[login to view URL](640): Mage_Core_Model_App->run(Array) #6 /home/domain/public_html/[login to view URL](80): Mage::run('', 'store') #7 {main} Furthermore, the reason this error came about is we enabled cache on the store to install a new module, no files were uploaded and then we went to compiler to make sure it was disabled as per instructions and the above error showed. Also when we access the downloader within magento it gives us an error. We need all of these fixed and the store back functioning. Please make sure you know Magento VERY well and can handle this project