The bitcoin.conf file doesn't exist. But I had a look in the apps settings;
There is a "prune block storage" setting?
http://prntscr.com/w0lz64Otherwise I can go to advanced settings config and it will create that file by the looks (but it opens empty).
Uncheck that and it'll disable pruning. The client will redownload all the blocks from the start and synchronize again. You'll thus be downloading about ~350GB worth of data as well as it's equivalent disk space.