If you are lucky, the wire is still continuous with the remainder of its coil and you can solder it back onto its terminal. I don't know the numbering system on that transformer (at least, I don't think I do), but the other end of that coil terminates on another terminal. A resistance check between the terminals that represent the ends of that particular coil should tell you if you have been successful. Hopefully, someone here will tell you which terminal number is at the other end.
If possible, try to clean the end of that loose wire with a bit of emery paper before you attempt to re-attach it. Be VERY gentle! The smaller the wire, the more likely it is to break!
This is not an impossible task, just a difficult one. Remember, somebody at the transformer factory had to do it originally, so it can be done.
Good luck!
Update: IF this is the same PT-3 as used in the SEX amp, the
measurement will be between terminals 9 & 10, and
will be approximately 80 Ohms.