The file probably got corrupted. Maybe you had two different instances of the editor open at once and tried to save the file causing an error.
Try going into some of your blueprints & look for compile errors that might be interfering with the map loading. (though it should load with a “bad” image icon on the BP)
Check whatever assets that use sprite trigger box and collisionComp then maybe they have compile errors and that may be stopping the file from loading.
If the map got corrupted then there is no real way to fix it (no tools). You would have to resort to a backup if you have one.
Loss of data is a hard lesson to learn. Look into data backup via a local repository either through git, github, perforce or many other version control software.