This happened to me when our LDAP connection broke between our JamfCloud instance and our AD server. After I got LDAP working again, JamfAdmin took my credentials again. If you aren't using LDAP, you may need to verify the Jamf permissions your account has.

Check your IDP configuration in the JAMF Connect Menu Bar app to make sure its right. If the config profile is right check with whoever keeps up with your IDP and make sure nothing has changed. Lastly make sure network traffic is good between the device and the IDP.

Uh oh... I just discovered that mine stoped working this morning. It was definitely working as of Monday and I am pretty sure it was working yesterday.  I verified all the usual suspects, but everything else is working correctly. @Unde0rwood and @Mouthbaten_1911 are you using JamfCloud or on-prem? We're on JamfCloud.

Make sure your account has access into JAMF Admin. 

I'm a Global Admin with full privileges and I just verified to make sure nothing was disabled on my account or group membership.

I was able to resolve our problem. Our security team made a change in the network that broke our LDAP connection - which prevented us from logging in to the JamfAdmin app. The security team fixed the change they made last night and now our LDAP connection and JamfAdmin are working.

As for the original question, I would ask "was it working before and then it stopped? or has it never worked?"

How can I confirm that?