You are right. It was because the disk quota was exceeded. Once I made more room in my account, simulation is running again.
Today I got another error message, which is shown below:
ERROR (ADE-3036): Errors encountered during simulation. The simulator run log has not been generated.
Possible cause could be an invalid command line option for the version of the simulator
you are running. Choose Setup->Environment and verify that the command line options
specified in the userCmdLineOption field are supported for the simulator.
Alternatively, run the simulator standalone using the runSimulation file in the netlist
directory to know the exact cause of the error.
What does this really mean? This simulation was correct and running fine previously. Today something is not right.