In 9.3.6 and later you can run the following commands on the Harvesters to reset the state of the router to allow it to collect data again:
mysql -unetqos -pnetqos harvester
update routers set pollState='InitialPoll', stateretry=0, reboottime=0 where pollstate ='RebootRefresh';
In 9.3.3 and earlier, this setting is stored in the poller database, so you can run the commands below on the Harvester servers:
mysql poller
update routers set State='InitialPoll', stateretry=0, reboottime=0 where state ='RebootRefresh';