What happenes, when at some time, all nodes on the blockchain go offline?
While the answer depends on the exact consensus protocol used by the blockchain, one can expect the following:
1. No transaction will be recorded while all nodes are offline.
2. Previous transactions (in the last few blocks) will not be finalized for longer period if ever.
3. An adversary may be able to gain control of the blockchain with less effort/resources than usual.
Will the blockchain be dead forever?
The answer depends upon
1. If the offline event is caused by network outage, then the blockchain will likely resume operation when the connectivity is restored.
2. If the nodes have lost interest in the blockchain, it may never come back to life.