Can't upload a sketch any more - board defective?

I'm learning Arduino with a starter pack (Sunfounder UNO board). I follow the lessons to learn the programming and components.

Up until last week everything went without problems. When I tried a new lesson I got the message "programmer is not responding" when uploading a new sketch. Even when I tried to upload a simple led sketch, it didn't work. I checked eveything: the board was choosen correct, I tried other ports in the IDE, I tried other USB ports and other USB cable on the computer,... nothing helped. I even tried another computer. It also didn't work.

At last I bought a new board. At first I tried a simple "led sketch" --> no problem Then I tried the same lesson when the problems started --> no problem I tried the old board again --> same problem again

So, I suppose the problem is the board? But what is/was the problem? Just a fault in the board? Or did I do something wrong with the last project? Some shortcut or something?

You may have had the board connected to the wrong COM port. I get that problem too. I constantly have to switch the board from one USB port to the next because the COM port is wrong. So your board may or may not still be working correctly.

Can you power the defective board and observe the behaviour of the "L" led. The led should blink three times repidly when powered up and/or being reset to signify the bootloader being ran.

If the "L" led does not blink as it should you can use your new board as an ISP programmer to reburn the bootloader. You can find the instructions forndoing thisnby searching google.