Urgent .NET Application Login Issue Fix

My small C# / ASP.NET web application, hosted on a Windows server (IIS), stopped allowing log-ins this morning. Instead of authenticating, it immediately throws the “Incorrect password” message for every user, even when the credentials are definitely right... (Budget: ₹100 - ₹400 INR, Jobs: .NET, ASP.NET MVC, Database Administration, Database Development, IIS, SQL)