Rebuild Windows cluster for Alwayson.
Questions to ask yourself start with:
How fast do you need it up?  Is it costing you money per hour being down?  Are you out of SLA?  If you don't have SLA's yet for this service can you get them real quick or have at least a good idea if it's OK to be down?

Typically you would want to learn why it is down then decide but sometimes time doesn't allow that.  Other's it could be within maintenance hours so if you know how long it takes to rebuild the cluster you know how much you have to troubleshoot.  

Thus my answer would be: figure it out as long as possible while giving yourself enough time to rebuild and test it while keeping the business need it serves in mind as priority.

Personally I try to manage a process and not servers.  I wrote a lot of Powershell automation to build up a brand new FCI from scratch, build out a read only replica AG node, test, open network, test network, etc. etc.  Something like this is great if you're rebuilding and you can start your automation that way if you haven't already.  I can give you some reference on how to begin if you'd like.

Let us know if you need more info by editing your question to give us more background and clarity. Thanks!