Hello b:

It will be considered a dirty UA.

excon

Yes, the PO will assume you were dirty and would not come in.
One of the worst things you can do, to turn a already over worked PO against you even more is to just not show up.

What's your excuse? Whatever it is other than being in a coma in a hospital the PO's not going to go for it and you'll more than likely be violated.

Probably get warrant for return to prison order