The answer by James K is pretty much correct for contract law, at least in common-law countries; I am not so sure about the civil-law position.
There are, however, situations in which an actual signature is required. For example when one issues a negotiable instrument, an actual signature is required and is binding, whether any contract is involved or not. Transferring a deed also requires an actual signature, not just an agreement, in many jurisdictions. In some jurisdictions, so does making a will.
In many jurisdictions these can now be done with some form of an electronic signature, and I don't know of any jurisdiction where a cryptographic signature is required for a valid digital signature. (A will is more likely to require a physical signature than most other documents, still.)
Even so, a forged signature is not valid, provided that the evidence shows that it was forged. A cryptographic signature made with a stolen key is forged. A person who revokes a cryptographic signature after using it would still be bound by it, provided that evidence can be presented to show that the authorized person actually made the signature. This would be true even if the person had, by mistake, used a previously revoked key. It is the signer's intent that matters. But others would be less willing to trust a revoked signature.
Making a signature and then revoking the key would not release the signer from an obligation.