Starting on 27 March 2015, a very large number of people have found that their Avios balance has been reset to zero. Their list of transactions shows an "Ex-Gratia" deduction of their entire previous balance. Other people are also reporting they are unable to access their accounts at all, with their BAEC number not being recognised.
BAEC call centre staff do not seem to have received a thorough briefing and are giving at times contradictory information. However, an email has gone out to some, not all, BAEC members affected by this issue, with the subject "Executive Club Password Change", details here in post 181. At present there is no definitive information about the exact cause, but it’s clear that BA believes there has been a serious security breach (or that there was a serious risk of such a breach).