First of all, thank you all so much. Now WMP is indexing my new files again.
I have one doubt: I did everything exactly as Jazacoop said, except that I only deleted the "LocalMLS_*.wmdb" files. I did this because I was afraid that if I deleted "CurrentDatabase_***.wmdb" too, I might lose all data related to the date added, which in my case is extremely important to access the recently added songs easily.
Again, thank you very much!