Cant start bitcoind - "Block files have previously been pruned"
The message says

2018-11-16T09:16:48Z LoadBlockIndexDB(): Block files have previously been pruned
2018-11-16T09:16:48Z : You need to rebuild the database using -reindex to go back to unpruned mode.  This will redownload the entire blockchain.
Please restart with -reindex or -reindex-chainstate to recover.

If you have been running in pruned mode, bitcoind has been pruning the blockchain. This means it has discarded data to save disk space.
To run in non-pruned mode you must recover the missing data. The method provided is to download the entire blockchain again.
The only obvious alternative is, presumably, to run in pruned mode.

Related questions:

How can I run bitcoind in pruning mode?

Related links:

https://bitcointalk.org/index.php?topic=1599458.0 (August 2016)


OK, I have a question. Say: I enabled prune mode and Bitcoin Core effectively started working in this mode. But it is one way ticket if I ever want to go back to 'full' mode I need to download and verify whole blockchain once again there is no shortcuts?

Yes, you will have to redownload the entire blockchain, there are no shortcuts