How to fix status: red in unassigned shards?

I got 20-30% of unassigned shards in a cluster and showing status red. How to Fix that issue.

Some nodes are down at certain point of time and they are OFF now for around 2 hrs. and now they are Up and showing status red

Please stop posting the same thread, you have asked this in 3 different ones - How to Fix Unallocated Shards?