Trouble Uploading to Arduino Uno

I have recently been having trouble uploading my programs to my Arduino Uno board using Windows 7 64 bit. I got it about a month ago and went through the entire Getting Started book without any issues. But now, when I try to upload a program to the board I get this error:

Binary sketch size: 1026 bytes (of a 32256 byte maximum)
processing.app.SerialNotFoundException: Serial port ‘COM9’ not found. Did you select the right one from the Tools > Serial Port menu?
at processing.app.Serial.(Serial.java:153)
at processing.app.Serial.(Serial.java:76)
at processing.app.debug.Uploader.flushSerialBuffer(Uploader.java:75)
at processing.app.debug.AvrdudeUploader.uploadViaBootloader(AvrdudeUploader.java:93)
at processing.app.debug.AvrdudeUploader.uploadUsingPreferences(AvrdudeUploader.java:56)
at processing.app.Sketch.upload(Sketch.java:1603)
at processing.app.Sketch.exportApplet(Sketch.java:1568)
at processing.app.Sketch.exportApplet(Sketch.java:1524)
at processing.app.Editor$DefaultExportAppHandler.run(Editor.java:2327)
at java.lang.Thread.run(Thread.java:619)

And when I try to go to the Tools menu, the Serial Port menu is grey and unselectable. I have gone through the troubleshooting section and it hasn’t helped. Thanks in advance for any help.