Cant find COM8

Development Tools IDE 1.x
1

It gives me this error msg

Binary sketch size: 2,418 bytes (of a 32,256 byte maximum)
processing.app.SerialNotFoundException: Serial port 'COM8' not found. Did you select the right one from the Tools > Serial Port menu?
at processing.app.Serial.(Serial.java:191)
at processing.app.Serial.(Serial.java:77)
at processing.app.debug.Uploader.flushSerialBuffer(Uploader.java:77)
at processing.app.debug.AvrdudeUploader.uploadViaBootloader(AvrdudeUploader.java:172)
at processing.app.debug.AvrdudeUploader.uploadUsingPreferences(AvrdudeUploader.java:67)
at processing.app.Sketch.upload(Sketch.java:1706)
at processing.app.Sketch.exportApplet(Sketch.java:1662)
at processing.app.Sketch.exportApplet(Sketch.java:1634)
at processing.app.Editor$DefaultExportHandler.run(Editor.java:2346)
at java.lang.Thread.run(Thread.java:619)

When I go to device manager, it tells me Arduino Uno R3 (COM8), so I dont know what to do. Please help me out !

2

Try simple first.
Unplug the Arduino
Reboot the computer
Plug in the Arduino
Start the Arduino IDE

Sometimes windows serial port handling can get flaky.

3

Well, if you are able to upload something sometimes, I suggest you to test http://arduino.cc/forum/index.php/topic,118440.0.html because the "wrapper" in avrdude will attempt to 'disable/reenable' the plugged arduinos on failure.

Otherwise, smeezekitty solution is the way to go first.