Basically my computer hard drive crashed -- luckily most of my stuff was backed up! Unfortunately, after redownloading ConvLib (which I never used in my game but I must have ticked a box somewhere or loaded it somewhere to make it work so now it's required), it's still failing to load the game due to the following errors:

*Invalid XML: For security reasons DTD is prohibited in this XML document. To enable DTD processing set the DtDProcessing property on XmlReaderSettings to Parse and pass the settings into XmlReader.Create method.

Any idea how I can fix this so I can get back to working on my old file?

Go here, right click the RAW-button, choose save link as and replace what you already have with what you just downloaded. Should work.

Another way you could try is to right click the game file and open it not in Quest but in a text editor (Notepad++ or similar). A few lines from the top you should see the code implementing the library into your game. Remove that line, save it and reopen the game.