Could you please ensure that the MongoDB service is running? You can do it executing the command:
/installdir/ctlscript.sh status mongodb
Please note that you have to replace "installdir" with the path to the directory where you installed your Stack.
If it is stopped, you can start it running the command below:
/installdir/ctlscript.sh start mongodb
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.