When a document is opened by a user, TopSolid locks it to prevent other users from also being able to open and modify it, which could have disastrous consequences because only the modifications of the last user who saved the document would be preserved.
The lock consists of a file with the same name as the opened document, but with the .lck extension. This file is automatically deleted when the document is saved.
If the software is shutdown unexpectedly (due to a power outage or other causes), this lock file is not deleted and it will block the next opening of the document. Therefore, you must delete it with Windows Explorer in order to open your document.
If you are working in single-user mode, this operating mode is not necessary and you can deactivate it by checking the None box in the Tools > Options > Document > Others command.