Prime was a one year plan, it was also discontinued sometime 2015 if I'm not mistaken, after that all accounts was in reality free accounts although the users that had an active subscription at the time it was discontinued was allowed to keep the perk of not having a login requirement.
This is now required after the migrations and in early march we removed all accounts that had seen no login activity in the past 60 days.
Due to there being no warning emails as those are part of the automated, currently halted, system we said we'd this time allow for a restore if requested within reasonable time.
Initially we said cutoff for restores would be one week, meaning in reality we should not restore anyone past Sunday 15th, though we extended it a little bit extra, but it was said that the last cutoff date would be on Sunday March 22nd.
You're lucky though that the restore is still possible for your account.
I'm running the restore now and the account should be good to go in a few minutes.