Hi,
So, my most important lightrooom catalog won't open and I don't know what to do. Screenshot below. I guess it didn't shut down before I ejected my hard drive.
Look in the backups folder, there should be several folders that are dated, look in the latest date and open that catalog, depending how much time was between the last back up and your last work, will determine how much work you lost.
What I recommend (hind site here) is once you get your catalog open is save a catalog first in a location easy to get to, normally where your old corrupted catalog was. then select all images then save. (ctrl/cmd-s) This will write the metadata to your files on the drive or side car files(depending how you set it up and if they are raw or jpg. ) Then you will have this for a backup, as you will know that all edits you have done are in the metadata, therefore you can create a new catalog and import all files to get back to where you were.
That said, use this method as a last case scenario, normally using a back up catalog would be good enough. However, if something happened to that main folder, like corrupted segments on the drive, you would have your images to fall back on, or at least those you can access.
But if your backing up to an external drive, you should be alright anyway.
I tend to go over board with backing up since I had a drive that crash a few years ago.