This error can be caused by a number of reasons but most common is due to the key register in the registry settings being incorrect.
You need to check event viewer and see what error is being populated under the secure module. It will give an idea what kind of issue is being faced.
Once problem has been established you will need to check the key register and also check the key file to confirm the registry details for the SGC.
NB: The customer might not have the keyfile and customer will need to advise further in terms of the changes made in the registry, if they still remember.
When investigation is done you will need to stop the secure module service and make the changes in the regisrty settings. Start the secure module service and test if issue has been resolved by doing a vend on a test meter.