There are several issues that can cause that type of "garbling", I'd recommend using Bing.com to search for specific types of events that can cause garbling. The two issues I've encountered more often - always from clients - are missing fonts (a font is used that does not exist on your computer OR an embedded font generated from a Windows Office app) or there are one or more embedded Tables in the Word document.
Word has Options for "Repair" and "Recover Text From Any Document" in the File>Open dialog - I suggest reading up on those two options and giving them a try on a copy of the document you're trying to open.
