you might have inadvertently put password protection in.
log in to your admin page at yourshop.myshopify.com/admin
go to “preferences” (top tab on right)
go to “general settings” (first tab on right nav bar)
scroll down, check “control access to your shop” → “password protection”
if you want to provide public access to your shop immediately, remove the password if there’s one in there.
click “apply settings”