If you only have a couple of deleted contacts to add back to your account you can just type or copy and paste them into your account as though you were adding them for the first time. Just keep in mind that their previous reporting and engagement history won't be available.
If you've removed an entire list you can either re-import the list information from a file or, if you don't have that information at hand, you can grab the most recent record of who was on that list from a previously sent email campaign.
Go to Emails > Reports.
Find the most recent email that you sent to the list you removed.
Click on the hyper-linked number in the "Sent" column. Note: If the email was sent more than 180 days ago, you might not see a hyper-linked number. That means the list information is no longer available for you to access.
Click the check box in the upper-left corner of your results to select everyone in that list.
Click Add to Email Lists.
Select the existing list you'd like to re-add these contacts to, or create a new list.