benger,
Sorry to here that. Pretty sure corruption is cause. Wonder if other games act same? Or if game itself is caught on something.
Just in case game config corruption is the cause I would delete %USERPROFILE%\Documents\BioWare\Mass Effect 3\ BIOGame\Config\GamerSettings.ini.
This will likely create a fresh ini file with default resolution.
Failing that, force Mass Effect 3 resolution using manual method (which is SoftTH fullscreen):
http://www.wsgf.org/blog/imusrt/2015/04/08/mass-effect-3-2012-manual-plp-instructionsEXCEPT, this change:
Under Notes, #4.B & C, regarding LUA edit: this whole step is not needed I think, if you use FWS detection override instead (like PLP, described in a previous post).