Nigel Foulkes-Nock explains why SSAS might not be available even if the service is running:
When starting SQL Server Analysis Services (SSAS) Tabular, the Service is quick to report that it has started. In my opinion, this Status is not entirely accurate – SSAS may be running but you cannot access data until it has loaded all associated SSAS Databases into memory and performed its consistency checks. This can take a long time.
After starting SSAS, if you try to browse the Databases using SQL Server Management Studio (SSMS) then SSMS becomes unresponsive. You will receive errors if you try to query a SSAS Database. It’s busy but it doesn’t report as such and doesn’t give any clue of how long it’ll take.
Read on for the explanation.