It would help to know a little bit more about what you were trying to do. Having said that, i had the same issue doing a minor upgrade in Moodle 2.9, and i solved by forcing the update via GIT, so you could try that. Normally you could also try to erase the moodledata dir (if it's a clean install it will most certainly fix the issue), or navigate in the Moodledata directory erasing the cache and session.
Hope that helps