There's a couple reasons why a client's contract could end. Maybe the client wants to cancel it, or they refuse to pay, so you want to end it yourself. To learn more about how to cancel or stop a contract, read this article.
It might also happen that a client changes their mind after you've cancelled the contract. Or you've just stopped a contract and the client decides to pay up again.
Our system unfortunately does not allow you to undo a cancellation. When a contract is cancelled, and the end date has not yet been reached, you can move the end date further up in the future, but the contract will keep the status "cancelled." A stopped contract is completely irreversible.
If you want to continue the contract, you will need to assign a new membership. When doing this, make sure you set the start date on the day the old contract ended. If there are still open payments from the last contract, you might want to set the start date accordingly.