The only question I am asking is what is calling the proc, from your answer I am guesing it is a program. What credentials are you using in the program to to access the db in the program. I am guessing it is different then your personal credentials.
Verify those credentials rights have not been alterred or that they are not locked. I had a similar issue a few years ago and chased it around for hours until I found out the user credentials used to access the db were locked out.