There is not a way to reopen a bank reconciliation once it is posted.  You will have to unreconcile the transactions that were posted to allow yourself to reconcile them again.  Below is the Knowledgebase article on how to unreconcile account transactions.

Unreconcile Account Transaction

Audience: This article is available to customers.

Answer ID: 25480

Solution

This knowledgebase solution provides information on how to unreconcile a transaction.

To unreconcile a reconciled account transaction, select the Reconciled items option when starting the account reconciliation. Clear the check box for the reconciled item, then post.

BusinessVision 2010 and Above

Click Utilities, then click Account Reconciliation.

In the Account No. box select account.

In the Reconciliation date box, select date of transaction.

Leave Opening and Closing balances boxes blank.

Select Reconciled items option.

Click OK.

Clear the check box for the required transaction(s).

Click Post.

BusinessVision 2009 and Below

Click Utilities, then click Account Reconciliation.

In the Account No. box select account.

Press Tab.

In the Statement date box, select date of transaction.

Leave Opening and Closing balances boxes blank.

Select Reconciled items option.

Click Ok.

Clear the check box for the required transaction(s).

Click Post.

The account transaction is now un reconciled and is available for reconciliation again.

Business Reason

This process is used when a transaction was incorrectly reconciled or to unreconcile a number of transactions so that an account reconciliation can be done again.