Time to collect all information you can - design notes, requirements specifications etc.

Then next step: Test, test, test to figure out what the current program does.

Then create the documentation that the company should already have had in a safe place.

Finally: Recreate the source code from the behaviour of the existing product with a few peeks into the assembler for unknown magic not possible to deduce from the black-box testing.

Tell your boss that a bank box is quite cheap and that the source code and documentation is important enough that they should be duplicated onto multiple CD and stored someplace where they can't be stolen or destroyed by fire or other accidents.

Luckily, you program is quite small, and the code to access the hardware can be easily duplicated from the processor datasheets and the hardware schematics. That leaves you with figuring out the busines logic.