The Google Analytics tracking code is not the type of code you use over and over. You usually just delete it once you are done installing Google Analytics on your website, without even thinking whether you are going to need it again in the future.
There are some scenarios you will need your Google Analytics tracking code again. Probably you have just installed a new theme on your WordPress site.
To install Google Analytics on WordPress, you usually paste the tracking code to either the footer.php file or the header.php file of your WordPress theme. So, if you want to change your WordPress theme, you need to paste the Google Analytics tracking code to the footer.php file or the header.php file of your new theme in order to have your Google Analytics keep works on your website.
You can find your Google Analytics tracking code from the Admin page. To go to the Admin page, log in to the Google Analytics dashboard and click the gear icon on the lower-left corner of the screen.
Once the Admin page opens, click Tracking Info on the Property column.
You can find the tracking code by clicking the Tracking Code link.