If you are using LinkedIn on a computer (Mac or PC), follow these steps:

You can manage your blocked list through your account settings on both desktop and mobile devices.

LinkedIn has a safety feature that prevents users from "block spamming." Once you unblock someone, you generally have to wait before you can block them again. If you are unblocking someone just to check their profile and plan to re-block them immediately, you will have to wait a few days to do so.