There is a known problem in older versions of GM with the file corrupting itself splat if the source gets too big. GM7 was supposed to fix this, but I long ago got used to keeping everything outside the exe and keeping the exe as small as possible.
HOWEVER, if you are able to open the source file in the editor without any problem and it's just RUNNING the game that's not working, it's probably fixable. Really, you should post on the GM forums - a lot of people there are idiots but there are folks who will try to help (and I can probably do better troubleshooting there than here).
If you're getting 'unexpected error' the first basic checks I would do are:
1. Have you added any very large image to the file recently? A lot of people try to load a ginormous background (something over 1600 pixels in one dimension) and find that their game explodes.
2. Can you make a new empty room your starting room and will that load up? It may be that there is some code in your existing starting room that is causing a kaboom.
Or you could send me the file and I could poke at it for you.
![Smile :)](./images/smilies/icon_smile.gif)
No guarantees obviously but I at least know a few places to look, I've been working with this program a long time.
And I assuming it's a bad sign when all the backup files get sequentially smaller in file size as they get more recent. :3 And I'm also assuming the fact that the project file is now 3 megabytes smaller than the one I had just a moment ago when I didn't add or subtract any new content is also a pretty bad sign. Yup... This is showing all the signs of a file corruption.
... It's not *always* a bad sign that the filesize is changing, it does vary somewhat because of the encryption. But dropping 3meg is a little worrying, yes, since it implies that either a pile of data went missing or your game is RIDICULOUSLY HUGE to begin with, which is just asking for it to corrupt...