If config cannot be loaded due to corruption or bug, it isn't handled gracefully (just stops)
If the current (and perhaps some recent) backup configs in the cache are unusable, due to failing checking on router startup, the router doesn't handle it gracefully at all.
It can reach shell but not GUI, and doesn't provide any help to identify which other backed up configs might be usable and what's changed since then which might explain the problem or allow console-based editing to fix it.
What I think should happen is that if this happens, it should try to identify the most recent working config, and the first config after that (if not the current config) which is probably where it "all went bad". It should then pop up a stub menu on the console advising the user that the config was bad and either no older working configs are held or that an older working config was found (and its name):
"pfSense was unable to load the current configuration, as the file is missing or corrupt."
"No older working config was found in the cache."
"An older working config was found in the cache but may be out of date. The filename is '***' and it is dated ***."
1) View recent working configs in the cache
2) View changes from latest working config to current config
3) View changes from latest working config to the next (non-working) config
4) Roll back to most recent (or any other) working config