Do you still have the Product Reviews installed? You should be able to export your reviews from the Product Reviews app.
And then you can import them into a different app. Here is a guide on how to do that with Kooee Reviews https://resources.kooee.reviews/en-US/move-from-shopify-product-reviews-to-kooee-reviews-321306
Once the reviews are imported, you then need to update your theme blocks to use the new review app theme blocks (not the Product Reviews theme blocks.)
We can help you get up and running on Kooee Reviews if you like. There is a free plan available. https://apps.shopify.com/kooee-reviews