You don't mention what condition prompts your question, so it's difficult to answer directly. Ability to ride a bike is stored in the implicit/procedural memory part of the brain, so cognatively speaking, riding a bike is possible even when explicit/episodic memory is impaired. That said, it would not be advisable if spatial orientation has been affected. If you can't drive a car due to problems with spatial orientation and/or reflex impairment then riding a bike is also not advisable. If you are seeing an occupational or other therapist they would be able to answer your question directly based on thier evaluation of where you currently are in your recovery.