Does any one know where I could get mor information on errors that I get from my .NET.

I got this error and i would like to know what my fault could be:

An unhandled exception of type 'System.Security.SecurityException' occurred in mscorlib.dll

Additional information: Request for the permission of type System.Security.Permissions.FileIOPermission, mscorlib, Version=1.0.3300.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 failed.

Today I tried to use some code I used (successful, and error-free) last week, I got that errror this time. Could someone help me please?