A few things to try...
1 Can you provide more details of what you are doing to flash the nxt (So what OS, what happens when you flash the nxt). Have you been running an older version of leJOS, or is this the first time you have tried it?
2. Can you reflash the Lego code and then try leJOS again.
3. leJOS may be running there may just be a problem with the display, so try the following after you have turned on the device and have a blank screen.
a) Press the Orange key and the square gray key at the same time. Does anything happen to the display (like does it flash a little).
b) If you are using windows unplug/plug the USB cable do you get any notification sounds from Windows?
c) Try to download a program. What happens?
I don't really know what flash does and how to do it. This is the second time i am using LEJOS, before I had 0.4 beta. I am using Xp Home. I don't know how to upload the lego firmware again because my lego program does not reconize my NXT. I have tried to flash about 5 times (but I may have done it wrong). The screen does not flash when I press the orange and tha gray buttons at the same time. I also don't get a noise from windows when I disconnect the USB cable. When I try to Download a program Eclipce shows
leJOS NXJ> Linking... leJOS NXJ> Uploading... error BluetoothStack not detected com.intel.bluetooth.BlueCoveImpl.<init>(BlueCoveImpl.java:136) an error occurred: Bluetooth stack not detected
Try doing a long reset of your NXT - press the reset button for 4 seconds. Check that you hear the pulsing noise from your NXT. Press the Orange button if you don't hear this at first.
Then see if Windows recognizes it as a USB device - it should make a noise when you connect and disconnect it and an entry should appear in Control Panel > System > Hardware > Device Manager. It should appear under Lego Devices as LEGO MINDSTORMS NXT Firmware Update Mode.
If this does not work, your NXT is not resetting. Try it again until it works pressing the Orange button to turn the NXT on first.
Once you have the NXT in firmware update mode, the Lego software should allow you to reflash the standard LEGO firmware, or you can try flashing leJOS again.
Make sure you are using a clean install of version 0.5 of leJOS and not an updated 0.4 installation.
I have my NXT in firmware upload mode, but my NXT's beeping stops after a few seconds. The computer found my NXT sp I tried reflashing the NXT-G firmware. The lego software said that it was downloading the firmware, then a alert comes up saying that there are communication erorr's with the NXT. After that lego has never reconized my NXT again.