Although it's deleted it will still be stored in the Database, unless you deleted it via the Database that is and if so you will need to create one via the Database, there is some SQL in the Installation Script, I'd suggest copying that if you have deleted it via the Database, I've put the script below for you
If you did this via the Admin Section (Never Actually looked to see if this is possible)
in dbo.Nop_Customer there is a column called Deleted = True or False if this is set to True you need to change this back to False
And you Should be able to log back in again
Else if deleted via SQL then the below SQL will recreate it.
INSERT [dbo].[Nop_Customer] ([CustomerID], [CustomerGUID], [Email], [Username], [PasswordHash], [SaltKey], [AffiliateID], [BillingAddressID], [ShippingAddressID], [LastPaymentMethodID], [LastAppliedCouponCode], [GiftCardCouponCodes], [CheckoutAttributes], [LanguageID], [CurrencyID], [TaxDisplayTypeID], [IsTaxExempt], [IsAdmin], [IsGuest], [IsForumModerator], [TotalForumPosts], [Signature], [AdminComment], [Active], [Deleted], [RegistrationDate], [TimeZoneID], [AvatarID], [DateOfBirth])
VALUES (37, N'18c4f7dc-2964-41db-9611-8a313bf672d3', N'
[email protected]', N'
[email protected]', N'14F009DDBEF203EDC2124D54FA6D325E16E36D77', N'vw7zHZI=', 0, 78, 77, 0, N'', N'', N'', 7, 1, 1, 0, 1, 0, 1, 0, N'', N'', 1, 0, CAST(0x00009B3600C2192F AS DateTime), N'', 0, CAST(0x0000769B00000000 AS DateTime))
Hope this Helps,
Rob