A bit of background here:
Lightroom itself is an application.
The catalog is a database that Lightroom relies on for most of its functionality.
Your images are separate again where the catalog holds references to the locations of your images along with other metadata related to each image - this includes editing information.
What has happened is the the catalog (database) has been corrupted.
The application itself is almost certainly fine with no issues.
There are many reasons why a database may become corrupted.
Sometimes the application controlling the database (Lightroom in this case) can repair the problem and sometimes, as in this case, it can't.
However, in this case Lightroom is offering you an option to try and repair the catalog again - I would go for this option initially, there is nothing to lose here...
It is likely however that this catalog is unfixable.
Possible options include going back to catalog backups - these are what Lightroom offers to do when it closes.
So, the question is: have you been allowing Lightroom to make catalog backups and when was the last backup done?
The other possibility depends on whether you have been backing up your catalog (and images), independent of what Lightroom does, onto different hard drives.
If you have, then it is simple matter of opening the most recent backup.
Perhaps you have been regularly writing metadata back to file.
In this case although not all the metadata that Lightroom can hold in the database is written back to file nearly all of it is.
And so, if you have to create a new catalog and import all your images from scratch Lightroom will use the metadata to reconstitute the database. This includes things like keywords and all the editing metadata.
However, it is possible, likely perhaps, that you have neither Lightroom-generated catalog backups nor independently-generated catalog backups, nor the foresight to save metadata back to file.
In this instance you will have to chalk this up to experience and start again with a new catalog and import all your images from scratch...
Whatever your situation here I make a strong suggestion that you rapidly learn all there is about Digital Asset Management as it pertains to digital image collections (and there is plenty of good information out there) so that in the future an occurrence such as this is simply an annoyance that can be overcome in a few minutes.
Tony Jay