The data is stored in memory before it's sent. If the indexer is down and you have not specified any explicit path to store the pending data that data will be gone.
You may want to take a look at persistent queue subject and i am gonna sleep.
ok thank you for the reply. it looks like I cannot use the persistent queue for my Windows events or the Monitor Stanza so is there any way to increase the 'in memory' queue for the universal forwarder?
if you see the document there it is mentioned how to configure the output in-memory queue. But as the secondary memory is more cheaper it should be used as persistent queue. For more in-memory queue length it should be configured as below.
maxQueueSize = [<integer>|<integer>[KB|MB|GB]]