To check if a file is being used by another program in Windows, you can follow these steps:
Open the Task Manager by pressing Ctrl+Shift+Esc or by right-clicking the taskbar and selecting "Task Manager."
In the Task Manager window, click on the "Details" tab.
Scroll down the list of processes to find the program that you think might be using the file. If you're not sure which program it is, look for any processes that are using a lot of CPU or memory.
Once you've identified the program, right-click on it and select "End task" to close it.
Alternatively, you can try closing any programs that you think might be using the file and then try accessing the file again in Minecraft. If you're still unable to access the file, it may be that the file is corrupt or that there is a permission issue.
You can try restarting your computer. Sometimes, files may be locked by the system and restarting your computer can release them.