A family of Microsoft relational database management systems designed for ease of use.
It might be you locking it - is tProjects open in a Form or Report, or locked by some VBA code?
If that's not it, close the database and see if there is a file in the folder with the database with the same filename but a .ldb extension. If so, this is a "locking" file left by some previous instance of Access which terminated abnormally. Delete the file and try again.