I accidently terminated an employee. how do I restore them so that all of their previous information comes back?

I'll be glad to walk you through the steps, @pamelagriffin.

You can change the employment status of your employee to make it active again. 

Here's how:

1. Go to Payroll and then choose Employees.
2. From the Active Employees drop-down, select Inactive Employees.
3. Tap the employee that you want to restore, and then tick the Pencil icon beside Employment.
4. Change the Status to Active.
5. Click Done.

Below are the screenshots of how it looks like.

To learn more about this feature, check out this guide: Terminate or change your employee's status on payroll.

Additionally, here's a link that'll guide you in case you need help with other payroll tasks: Browse all articles for QuickBooks Online Payroll.

Please let me know if you have follow-up questions or concerns. I'll be right here for you. Have a nice day ahead.