SOLVED!! Uber n0ob friendly!!!
Appears to be a multi-monitor problem / 'remembered position' issue as the other person suggested. But alas I have come to you aid. Lol jkjk probably fixed it for now but there are a few of these same posts and not all of them are answered.
2A -SIMPLE FIX- #2A
*** We will start by making sure Serial Monitor (not Arduino IDE) is the selected windows process ***
- Click the serial monitor icon in the windows taskbar or navigate to it from Task Manager.
2A. For Simplicity, after making the Serial Monitor the active window / process, try hitting 'WINKEY + LEFT' or 'WINKEY + RIGHT' ----------- (WIN 7 AND ABOVE) This is a neat little trick to fit the window to corners of your screen / navigate through monitors [check out WINKEY + UP/DOWN when you finish up]
IF #2A DOESN'T WORK
2B. 'ALT + TAB' release. Single press of 'm' on the keyboard then 'Enter'. Proceed to move the monitor back on screen with the arrow keys. ---- here we are selecting the alternative menu and selecting 'm'ove from the list; hence your single 'm' key press..
The taskbar in windows 7 and above may have similar items grouped, if this is the case you can simply find the serial monitor's icon hiding behind the Arduino IDE.
I also mentioned 'Switch To' from the 'Applications' tab of 'Windows Task Manager'
(CTRL + ALT + DEL) or with one less step (CTRL + LSHIFT + ESC)
Click once on the serial monitor then click switch to.