Short answer - if the function was working before, RESTART YOUR COMPUTER.
Long answer - I had this same thing happen to me. The problem is that the function I had created had been working for months. Then one day it just started showing a #NAME error instead of working like it was before. I had tried closing all other excel workbooks and even closing excel all-together and re-opening the sheet. Nothing seemed to work. Then for kicks, I edited the code to where I knew VBA would complain that there is a compile error. Surprisingly, it didn't complain. OK... I saved and closed excel anyways and then restarted my computer.
Once rebooted, I re-opened the excel workbook. Then VBA finally gave me a compile error. So I changed my function back to the original code I had before and now the sheet is running the function like it is supposed to. No more #NAME error.
Not sure all of those steps are necessary, but simply restarting the computer seems to have fixed my issue.