The usual reasons
You, or someone else, signed in on another device or browser. Your account stays signed in on one device at a time.
You were inactive for a while, so the member area signed you out automatically.
You changed your email address or password. For your security, previous sessions are signed out.
Your browser cookies or site data were cleared, which also signs you out.
If it was you
Sign in again. Nothing is lost on your account; only unsaved work on the device that was signed out.
If it was not you
Sign in again and change your password from the Security card on your Profile, or use Reset Password on the sign-in page.
Open Activity and look for sign-ins or "Signed out by another device" entries you do not recognise.
Contact [email protected] straight away (Terms of Service, Sections 5 and 6).
Too many attempts
To protect accounts, sign-in and sign-up attempts are limited. After several failed attempts you see "Too many attempts. Please wait a moment and try again." Access may be locked temporarily, and the lock grows longer if failed attempts continue.