You will have to change some of the HADR settings on the standby after you restore the primary database on the standby server. The local and remote host names are reversed, as are HADR ports, etc.
Next time, don't stop HADR. Just deactivate the standby database and then stop the instance. Do the reverse when you bring it back on-line (you may have to restart db as standby). Should be OK if you the outage period is not too long (the primary still has all the logs and can replay them).