The worst would be disciplinary action, certificate suspension, etc. But realistically, as a student, you would be given additional training. Your CFI, if he was acting as PIC (he had valid medical, license, etc) would be liable for any FAA actions. If you were acting as PIC because your CFI did NOT have the medical, then you may be considered as PIC, and would then be liable.

As a former DPE, I've seen a few of these actions in the past. The FAA usually will be satisfied with the PIC taking remedial training. But in the future, I would not talk to the FAA without first consulting an aviation attorney. And as another poster said, fill out a NASA report immediately!