File is used by another user

When tring to open an assembly file located on my hard drive,I get a message that the file "...is being used by another user. Doyou want to open a copy?" It will open a copy in Read-Only. Somebackground: I was working with this large assembly on the networkbut sometimes it gets slow. So I did a Pack and Go to my harddrive. I was able to open it and do some work on this file beforethe holidays. Came in this morning and opened the file but got thismessage.

Thanks in advance for any insight!SolidworksAssemblies