I had to go to a drug couselor as a condition of my probation. They assigned you a color, and you had to call in everyday to see if your color came up. If it did, you had to come in the next day to give a sample. In that program, you actually had three fuck-ups until they booted you.
If it is a PO, it depends on the person. Did you even call & leave a message? If not, I would do that immediately, and let your PO know that you have nothing to hide, and will be in immediately to make the test up, or at the PO's next convenience. I'd say some shit like "dude, I swear, I'll even take a hair-sample-test if you want me to. I am totally clean, and have nothing to hide. I just got tied up with last minute shopping is all, and I apologize". If you have been straight with the dude up to this point, then the PO should be understanding. Trust me bro, they have guys out on parole for rape and other sick shit, who they have to worry about alot more than you. However, if you continue(not saying you have..just saying) to take advantage of the PO's trust and intelligence, then they will have no choice, than to fuck you up.