Mischa Barton -- Screw Your Movie ... I'm Partying My Ass Off!

Mischa Barton is one step away from being featured on a milk carton ... which makes it especially stunning she blew off a movie role because she thought it would be more fun to party in Europe.

Mischa hasn't had a hit since "The O.C." wrapped in 2006 ... but she got a huge break when she was hired to star in the movie, "Promoted."

Producers claim in a new lawsuit -- obtained by TMZ -- they attempted to contact Mischa about the March 4 start date and also about sizing her costumes ... but got no response.

On March 3 ... producers got an email from Mischa's mom, saying, "Mischa is still in Europe and won't be available until the 20th of this month."

The producers -- furious -- went on Mischa's social media accounts to find she was actually partying in Europe ... on vacation and "enjoying tourist excursions with friends."

Mischa's absence created a snowball effect ... the lead actor didn't want to work with another actress, and so on and so on.

The "actress" is now being sued for the $20,000 advance she got, $200,000 in marketing expenses, and $100,000 which they say they will lose because Mischa's not attached anymore.