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.
These instructions currently apply only to the Megaventory accounts that are integrated with Magento. There is no way to undelete products through the Megaventory user interface at the moment.