Liz here, a Shopify Guru. :)
The Shopify Lite plan allows you to still sell products on an external website or other sales channels like Facebook or Amazon and manage your sales on the Shopify Admin however it does also disable the Online Store channel. To have the online store (to essentially host your website on Shopify) you would need to be on the $29 USD Basic plan or higher.
To get your Online store back, head over to settings > sales channels and click Add Sales Channel. Once you've selected the online store it will prompt you to choose the higher $29 plan.
Let me know if you have any questions about the process! I'm happy to help.