Not all paintings and other works of art will be signed. They may have been kept in an artist’s private collection or simply weren’t for sale during his/her lifetime. It isn’t uncommon for paintings to be left unsigned, especially with Impressionist artists. Even Rosa Bonheur, a well-known French realist who was popular for her animal paintings, had many unsigned works at the time of her death, and a highly-publicized studio sale was conducted to sell them.
If your painting is unsigned, it may still be valuable. There may be an estate stamp or some other clue to indicate where the painting came from and when it was produced. A trained art or antique dealer can help you determine the value.