Dallas here from the Social Care team here at Shopify.
When it comes to orders with Oberlo and Shopify it is important that the connections between the products are connected properly. Any adjustments with the variants/SKU numbers can affect the connection with Shopify & Oberlo. Have you made any adjustments with this in the past little bit? It is also possible that the product isn't available on Oberlo anymore.
If you go to Apps > Oberlo in your Shopify Store Admin you will be taken to the Oberlo page. When you click on My Products you will be shown a list of all of the products that you have attached to your store. From there you can take a look for the product that you are having issues with to see if it is still available.
If this doesn't solve the problem please let me know. Also, if you are receiving any errors please let me know that as well. If so, can you share screenshots that would be wonderful as I may be able to direct my information specifically.
Thank you so much,