If you lack certain fields or assign incorrect names to products you will very likely receive error messages, preventing the sheet from being loaded. Lightspeed eCom uses .csv or .xml files. To import products of your previous store, you will have to convert its import sheet to the Lightspeed eCom sheet structure.
Copy the fields from the import sheet of your previous store to the template sheet. When you have finished, you may import it through SYSTEM > Tools > Imports in the left menu of the back office. For more information about importing and an explanation on the columns used in the sheet please see the Import support page.