If I understand correctly, you are saying after updating Acrobat Standard with the usual update not upgraded your application, you are not able to sign a document or stamp it.
If the above is correct, first thing i would mention. Open the pdf file and click on File>Properties> Security tab. Check if you are allowed to make changes to the file. Assuming you are using a windows machine.
Also, let me know the operating system you are using and the version of the application you have? If the above suggestion doesn't make a difference. Let me know if you are getting any error message while working with a file.
Also, please make sure you are opening the file in Acrobat, not in Reader.