I'm trying to reverse engineer a C# file that requires a password to continue running. When I load it in IDA Pro I get nothing in the functions window and a sp-analysis failed
If this is C# (or any other .NET file) you need to use a tools such as .NET reflector to "decompile" it into .NET intermediate language.
The error you are seeing means that at the end of a function the stack pointer (typically the ESP register) does not point to the address expected by the function type, for example: it may have
PUSHed something on the stack and did not
POP it by the end of the function.
This warning does not stop analysis, but you should be aware of the situation when
JMPing between addresses.
In general settings you can choose the "show stack pointer" options and see the state of the stack.