After writing an entry, I click on "Save" and then this message appears:

An error occurred:
Can't call method "title" on an undefined value at lib/MT/App/CMS.pm line 2146.

I don't know how I can go around to fixing that. I wrote an entry not too long ago and then suddenly, this started happening, so now I can't add new entries.

Also, on my weblog page, when I click on my most recent entry's comments, a new window opens and it says:

An error occurred:

Invalid entry ID '32'

Please let me know how I can fix these problems. Thanks!

Let me guess, you're using BerkeleyDB...

I'm thinking your entries database or its index has become corrupted. Unfortunately, there's not a lot you can do to fix a broke BerkDB.

Ah, does that mean I would have to start all over?

You can try to export as much as possible, if the database will let you. Also, link your templates to files and save the templates. That way, you'll have a copy of them in case the worst case is true.

If you have access to the command line, you can always attempt a db_dump and db_load to see if the table files will rebuild. See the appropriate "man" pages for those commands.