Macros in worksheet

I have a worksheet that contains numerous macros that normally works just fine. However, there are times when I try to use the worksheet and I get the error message saying that "the worksheet is already open and two worksheets with the same name can not be opened at the same time". Even though there is only one worksheet open, the computer thinks there is more than one and won't let me do anything on the worksheet. I end up having to delete the file from my computer and restoring it from my backup disk. Why is this happeneing?