To restore your old MetaMask accounts, you need to have backed up your 12-word seed phrase.
When setting up MetaMask for the first time, it has a button for "Restore from seed phrase", but if you missed that part, you can still do it by these steps:
- Select "Lock" in the top right menu.
- Click "Restore from seed phrase"
- Enter your 12-word seed phrase and a new password.
- Click restore.
- You will only have one account restored at first, click "Create account" (top right account menu) for any missing accounts.