Run-time Error '9': Subscript out of Range

I have some VERY basic code:

Workbooks("testbook").Activate

I am getting the error at this line and it's even the first line in my coding. This is the only workbook I have opened at the moment. I have checked over and over again...I know this is the right way to write the code...Based on some research, I believe I might be opening the excel file as an instance. However, this is the first time I've ever heard of an "instance". Why is Excel not recognizing my workbook object? This doesn't make any sense! Please HELP!!!

Hey Arlette -

No, that didnt work..