Find a lens and check every wall to make sure that you built it. Also make sure that none of your walls are touching anyone else’s walls, and that you have floor tiles covering every piece of visible ground within your walls. If you find someone else has put walls in, that’s probably who raided you. If you didn’t have floor tiles down then it could have been anyone, though if they left anything behind you can try using a lens to see if they picked it up at any point and then put it back down.
It happens, and you may never figure out who raided you either. Thankfully its not too hard to get started again; get a pickaxe and a hatchet, mine rocks in the desert for gold, mine rocks in the arctic for obsidian, and set up a few towers to prevent future issues.