I believe if the server goes down during a task being run then it will attempt to complete that tasks on resumption although I havent seen this in action.
In regards to scheduled tasks which should have gone whilst the server was down then no. NPrinting wont store them up for sending on mass when the server is next available to do so.
Imagine if you had a daily task and it was down for a week. 1. you'd overload users with out of date reports and 2. the server would be to busy sending these reports to send out the next available reports in a timely manner. You would need to handle the missing reports as exceptions and make the call on which are still relevant to send.
What version are you running? I've experienced in the version previous september release that if the server goes down during a scheduled task it starts over when the server is back up again meaning that if any reports was sent out they will be sent out again. This was confirmed a bug I believe but I haven't tested it in the latest november release. The easiest way to test I guess is to start a task, restart the server, and see what has happened? (Maybe without email destination being ticked)