Hi
The reference to 16.5 suggests you might be using AccountEdge. You can check your version by opening the sample file. If the sample file also can't open, then the problem might be with your computer/installation of MYOB/AccountEdge - try re starting the computer first. If however sample file opens, this narrows the problem to your company file.
From what you are describing sounds like you have a file corruption. One corruption can quickly lead to another and another if you keep using the file. If Optimisation can't fix it, repair or restore backup are the only 2 options.
Upgrading the file won't fix the corruption.
Regards
Gavin