Half a year of work ruined.!!!
Try Copying all of the objects to a new map file, if the issue is only that it crashes when deleting then there’s other options
Or go back to 4.22. Surely you have a backup before you converted to 4.23. Also do a code build or download the pdbs to find out where it’s crashing.
I also get a crash in 4.23 (Assertion failed), but not in working with a level 4.23 Crashes when moving assets - UE4 AnswerHub
4.23 is also crashing FREQUENTLY when I try to work in a level. Most likely when deleting an actor or even pressing Ctrl+E to edit a selected Blueprint after I deleted an actor just before that action. It is so annoying, however it didn’t ruin my project. I have reported all crashes since then and hope that Epic fixes them as soon as possible - to be honest, I can’t imagine how an unstable build like that could pass the QA checks at all