I upgraded to Win11 and it seems to me that this makes some problems.
With Win10 I could select different boards and all worked well.
Now I wanted to upload a sketch to an Uno with IDE 2.3.7 and get following error:
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x00
Now COM 4 is selected for Uno which disappears when I disconnect the USB connector. Therefore COM 4 should be ok. With Win10 it always was COM 3.
Device Manager tells me that COM 4 is working properly.
What can I do?
Windows11 might/will change the COMM port number.
I need to select BOARD and COMM PORT almost every time I restart the IDE.
I have a Lenovo laptop running Win11, IDE 2.3.7, with one available USB port. Depending on what I am doing (multiple sketches open) or what I have done, that COMM port has been assigned to COM3, COM4, (I don't think COM5) and COM6.
Did you try reset double tap, have you killed all IDE monitors?
Do you mean reset on the UNO board?
I am using IDE 2.3.7. There is only one sketch window open. In the lower part of that window there are two taps, Output and Serial Monitor. Of course I need Serial Monitor because the sketch to be uploaded sends data via Serial.print(). All the years I’m using Arduino IDE I never killed Serial Monitor before uploading a sketch and there were no problems at all.
I did a lot of research and tests. Now it works! I don’t know why before not and now ok. Hopefully this working status keeps on.
That was not the point. There is an issue in IDE 2.x where it can't upload to boards with FTDI232 serial-to-usb converter if the serial monitor is open. So close the serial monitor, upload and open the serial monitor again.
Because you did not provide any details in the opening post about the board @sonofcy's suggestion was a sensible one.
Please mark your topic as solved by clicking the solution checkbox under the most useful reply.
In all fairness, first post, third line:
Now I wanted to upload a sketch to an Uno with IDE 2.3.7 and get following error:
Very common, something you did changed the scenario. OR windows lost it's mind then fixed itself. All very common. When things like that happen, I try many things, restart the IDE, and seldom do the OS cuz I have a Mac and it doesn't have problems like Windows.
Did you re-stat the IDE?
I bet you restarted the IDE, so post 3 was the solution. Common issue.
Thanks for pointing that out.
I apologise to OP; I did read the opening post a few times, including once before I posted the reply and did not see it 
I restarted not only the IDE but also my PC, several times.