The basic process is to Deactivate the account(s) in question for
downloading, then Re-Activate them.
To reactivate, I recommend using the Add Account process.
When Quicken "discovers" your accounts at the financial
institution, you should see each listed in a dialog that allows you to
"Add", "Link" or "Do not download" each discovered account.
Make sure you "Link" each discovered account (that you are trying to
Re-Activate) to the correct existing Quicken account (one you
Deactivated before you started the Add Account process).