I've increasingly had issues getting Excel to open other Excel files if I already had one open. I noticed it happened every time I was working in one of my spreadsheets that contained macros. However after some research, I discovered that Microsoft has intentionally designed this so that if Excel thinks you are editing a cell, it will not allow you to open any other Excel files (even if they are unrelated).
Although there isn't really a true solution, if you hit enter, or simply get out of the cell as if you're editing it, or hit save, you should be able to open other files.