user72537: My serial monitor window opens frozen, without any output or possibility to input, or even activate the window.
Sketch is very simple, but behavior is same even with empty sketch, so I assume problem is not in sketch. I also checked if sketch is uploaded correctly by uploading simple sketch with blinking diode, which worked.
Same behavior with Arduino Nano and Uno.
I tried to reinstall IDE, uninstalled, removed and reinstalled serial driver (ch340) with no success.
I can see Serial monitor window only on windows taskbar, it won't activate after click.
Until now I haven't had any significant problems with IDE or serial monitor for years.


user72542: Try updating the java. Also check whether the device manager shows the USB serial port without any yellow warning sign on it.
If there is a current short in the usb port that specific port will be off for safety, restarting the computer should resolve that problem.

user72537: Unfortunaltely, nothing worked, and after hours of trying and googling I reinstalled windows, and now Serial Monitor works correctly.

72537: <p>My serial monitor window opens frozen, without any output or possibility to input, or even activate the window.
Sketch is very simple, but behavior is same even with empty sketch, so I assume problem is not in sketch. I also checked if sketch is uploaded correctly by uploading simple sketch with blinking diode, which worked.</p>
<p>Same behavior with Arduino Nano and Uno.</p>
<p>I tried to reinstall IDE, uninstalled, removed and reinstalled serial driver (ch340) with no success.</p>
<p>I can see Serial monitor window only on windows taskbar, it won't activate after click.</p>
<p>Until now I haven't had any significant problems with IDE or serial monitor for years.</p>
<p><a href="https://i.stack.imgur.com/Taibi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Taibi.png" alt="window state" /></a></p>
 72542: <p>Try updating the java. Also check whether the device manager shows the USB serial port without any yellow warning sign on it.
If there is a current short in the usb port that specific port will be off for safety, restarting the computer should resolve that problem.</p>
 72537: <p>Unfortunaltely, nothing worked, and after hours of trying and googling I reinstalled windows, and now Serial Monitor works correctly.</p>
