Easy solution would be:
When you are creating or editing post, in the tags section of WordPress (at-the-right-side), try typing name of the tag that you were using before and WordPress will popup showing existing tags.
If you don't remember tags you were using before and would like to add the same tag to the new one or other existing posts,
* Login to WordPress admin (yourwebsite.com/wp-admin/) >>
* Posts >>
Look for the existing tags, select, copy and paste tag to the post that you like to use.
If there are lots of tags, try search tags. Then click on Add.