It could be the existing signature certificate is expired or about to expire. Add a new signature certificate then delete the old one.
In Adobe, Tools => Sign & Certify => More Sign & Certify => Security Settings.
This opens a screen with two windows. The upper window shows existing certificates, the lower window shows the certificate info including dates Valid from and Valid to. In the top window, choose Add ID and follow the prompts to add a new certificate ID. This will create a new signature with new Valid from and Valid to dates beginning on the date the cert is created and ending 5 years later. Once the new cert is created click Refresh on the top window. After creating the new ID, close the program, then reopen and navagate back to the same page. In the top window, delete the old signature ID. The newly created signature ID with new Valid from and Valid to dates becomes the default signature. Close the window and you should be able to again sign documents. This worked for me.