Check your Gmail security settings
See the topic https://mangolassi.it/topic/12705/google-accounts-being-signed-out/ for further discussion
Updated A substantial number of Gmail users have been affected by a potential but unconfirmed hack of unknown origin or purpose.
El Reg learnt of the issue following a tip from a self-described "very security conscious" IT professional who got locked out of his Gmail account. This happened after one of his security phone numbers was changed.
Apparently others have suffered somewhat similar problems and have posted their experiences to Reddit and elsewhere. Users are receiving messages saying that their account has been changed, and asking them to re-sign into Google on their mobile. It's not clear if some sort of glitch or a hack is to blame.
This is more a case of being bounced out of accounts than being locked out as such.
In response to a thread on one of its official forums, Google said it was investigating the issue while downplaying concerns.