I can't upload my program to Arduino Uno I've not used my Arduino for about 2 months. Before this time it worked fine, but now when I want to upload my program I get this error: 


  Couldn't find a Board on the selected port. Check that you have the correct port selected.  If it is correct, try pressing the board's reset button after initiating the upload.


I reset immediately after uploading but it doesn't work.What else should I do?
I'm using Ubuntu and the Arduino IDE.
Checks:


Check if the board has power (probably it will).
Check if the board is shown at the ports in the IDE (i.e. COM3: Arduino Uno).
Check if you put the correct board in the IDE (e.g. Uno, Mega etc.).
Check if you have multiple versions of the IDE running.
Check if you have multiple IDEs installed.
Check if you don't have the RX pin used while uploading a sketch.


Try the following:


Use a different USB port on your computer
Use a different USB cable
Reinstall the Arduino IDE