Of corse, this can be caused by many different things, but if you’re stuck, without any ideas what could be wrong, than try solution bellow. Compiler stores everything in one file and can cause problems with includes. Just clear compiler and disable it if you don’t need it of corse.
I was getting error like this:
Warning: include(Mage.php): failed to open stream: No such file or directory in /my/web/server/root/public_html/lib/Varien/Autoload.php on line 94 Warning: include(): Failed opening 'Mage.php' for inclusion (include_path='/my/web/server/root/public_html/app/code/local:/my/web/server/root/public_html/app/code/community:/my/web/server/root/public_html/app/code/core:/my/web/server/root/public_html/lib:.:/usr/local/share/pear') in /my/web/server/root/public_html/lib/Varien/Autoload.php on line 94 Fatal error: Class 'Mage' not found in /my/web/server/root/public_html/app/code/core/Mage/Core/functions.php on line 244
Just login into your shell and then execute commands bellow (without comments):
//clear compiler php compiler.php clear //disable compiler php compiler.php disable
It is most likely that this will fix your problem. If not, keep digging 🙂
Hope it helps.