You need to create a new electronic signature to go with a new password. The old signature is toast.
You probably mean signing certificate. To change a password on the signing certificate you need to know the old password, which you do not have. How to recover from this problem depends on how you got your signing certificate. If you procured it from a Certificate Authority (CA), you need to tell CA to revoke your old certificate and to issue you a new one. If you created a self-signed signing certificate in Acrobat, create a new one and make sure that you choose a password that you can remember. It is generally a bad idea (security-wise) to write down any password.