I'll help you make the account active and add the opening balance. Please take note that QuickBooks won't add transactions back to the account or edit any journal entries it made to balance your accounts.
To reactivate the account, here's how:
Go to Accounting> Chart of accounts.
Click the Gear icon above the Action column and click select Include inactive.
Find the account you want to use again and hit the Make active link.
Please see this sample screenshot for a visual guide: