The message means that an assertion failed. Assertions are a defensive programming technique whereby the programmer states "the fact X must be true". If X is in fact true then nothing happens and things carry on. If X is false an error/exception is raised. If this exception is not trapped and handled it results in the program being terminated.
There could be a great many reasons for this and without a more detailed error message (which it would have already given to you if it was going to) or access to the source it would be very difficult to diagnose further.
One likely cause is an update to a system library that has changed (perhaps as part of a security fix) some property that the program relies on.
Have you upgraded your Windows version recently? If so, try running the program in the older Windows variant in a VM (for instance if you have Windows 7 Pro try using its "XP mode" feature, otherwise try install an older Windows in a VM using VMWare or VirtualBox). If not then you might be out of luck because if the problem is caused by a security update that update may be one that the older Windows variant will get also.
You may unfortunately have no choice but to either purchase an upgrade or find an alternative peice of software that fits your needs and can read your existing documents.