I accidentally removed:
$project = 'mysite';
from my _config.php and did a dev/build.
it gave me the "$project isn't set" error and when I added back in I accidentally set it to something else.
when i realise and set it back to "mysite" the dev build now works again but isn't detecting any of my custom classes such as blog or custom page types. both front-end and admin are blank. database seems to be untouched.