Hi Evie,
Greetings...!!
Thank you for posting. If you are the only user who is not able to access Yammer, please check with the Global admin if you have been assigned Yammer license.
If you were inactive in Yammer for more than a month, you would have been suspended automatically and please ask your Global/Verified admin to reactivate your account (Yammer-> Network admin-> Remove user-> Reactivate account if present in suspended list)
If the above does not work, please raise a service request with Microsoft Yammer where we would help resolve your issue. Please revert for any query regarding Yammer.
Thanks and Regards,
Sreejith
Microsoft Yammer Support