Just moved from a 2 monitor to a single monitor system?
The easiest way is to simple rename your 3dsmax.ini. You'll lose some preferences you've set, but you'll get your dialogs back. No need to reinstall - that should be a last resort after exhausting all other options. See
this post for how to find 3dsmax.ini.
Another option would be to
Make the problem dialog "current" i.e. click the Material Editor button on the toolbar (and dont click anything else!)
Press Alt+Space, then "M", then use the cursor keys to bring the dialog back into view, then press Enter.
Downside with this method is you'll have to do each dialog separately.
Note - this question has been asked, and answered, many times before. You may well find that searching these forums first would have saved you a lot of time and trouble.
You should also put your Max version, and brief system specs, in your sig - saves us having to ask when the problem is version specific. In addition, posting in the correct forum helps - this isn't really an installation/hardware/OS question.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).