digital signature stopped working.

i tried to sign a document but when i hit the sign button i get the following: "The Windows Cryptographic Service Provider reported an error: Key not valid for use in specified state. Error Code: 2148073483"  I have been using this signature for a couple of years.i checked "info" on the "Sign Document" page and the pop-up states "This is a self-signed certificate. The selected path is valid.The path validation checks were done as of 2016/01/11 ..."  I need fix this ASAP.