The Annual Allowance (AA) for pension contributions is now capped at £50,000. But your AA cap can be expanded by unused AA carried forward from the three immediately preceding tax years, if you were member of a UK registered pension scheme for at least one day in the earlier tax year.

Frozen rights in say a Civil Service pension scheme would make you a member of that Civil Service scheme. If you paid into a pension scheme many years ago, but have made no contributions since, as long as that old pension scheme is open to contributions your will still be a member of that scheme.

HMRC guidance on carry forward of annual allowance

http://www.hmrc.gov.uk/pensionschemes/annual-allowance/carry-forward.htm