Either you have it already opened it or previous time closing the project was done by killing it via tasks or a laptop crash.
First situation; dont open it a second time
Second situation; open it any way
If Rene’s good tips do not work and you get the error-message: it has to do with svn which is the Mendix’ version management tool. Try to first commit your code changes, or update the project.
As a last easy solution: remove your local code (or rename it’s directory) and redownload your project.