I am getting a serious application error whenever I try to save my .mxd and it immediately crashes. I have tried the "Save As" and that doesn't work either. I the tried to delete the .mxt file, and that doesn't work. When I go to ArcCatalog in my file structure I can find the .mxd but it is blank. What can be hanging it up? All comments will be tried.
You're gonna need to provide some system information. What version of ArcMap is installed? Are you running Windows 7, 8, 10...? Have you tried saving a different mxd? If your data is in a gdb try loading a shapefile and saving. Or try saving to a different location. Maybe try running mxd doctor, Using the MXD Doctor utility—Help | ArcGIS for Desktop
I hope this helps,