How to re-edit a photo after exporting it from Lightroom

Export makes a copy from a working image. The working image consists of an imported source file, and a set of  non-destructive edits that are previewed, live and dynamically, in relation to that source. If you want to further develop those edits later, including after exporting a separate copy, then you can.

But this is not a matter of editing the export you've made. It is a matter of returning to the image that this export was made FROM. As needed, you can later make another export which reflects your additional work.

As a matter of clarity, and of not confusing working images vs exported images, I recommend against having exported image copies inside the Catalog. But if you did reimport those, since they have never undergone any editing of their own yet - being brand new - they show all adjustments at zero.

If it helps, you can think of the imported file as the composer's musical score for a symphony. This official published score never changes. You can think of the working image inside Lightroom as the conductor's annotated copy of this published score covered in rehearsal notes, developing over time, regarding how they like to perform this symphony. And you can think of a given export, as one of those performances. To get a different performance next time, the composer's score doesn't change. The previous performance doesn't change. It is the conductor's adjustment of what they do, which leads to this different outcome.  

Editing tools are never locked, but they will be dimmed if the original image is missing. Did you perhaps delete it from disk (but not from Lightroom) thinking that you wouldn't need it anymore?

I opened Lightroom up again and it brought me back to my photo that I wanted to re-edit but all of the editing tools are locked.

By @Lilian29947939mjef

This really doesn't make any sense. Locked how? You need to show a screenshot of this.

But if you're not happy with the result, why don't you just go back to the original and do it the way you want? Depending on circumstances, further editing on the exported file may not give you the best quality result. Redoing the original is much better, especially if that original is a raw file.