When you delete a product from your Megaventory account it is possible to undelete them through your Magento backend.
To do this, in your Magento backend go to Catalog > Manage Products open the product you wish to undelete and click Save.
If the product is deleted in Megaventory, you will see a notification that will prompt you to undelete it:
You simply need to click the link in the notification to restore the product in Megaventory.
A similar approach is possible for product categories.
Disclaimer:
These instructions currently apply only to the Megaventory accounts that are integrated with Magento. To undelete products or product categories through the Megaventory user interface follow these instructions.