HP sb21 lamp

Try taking the lamp out and reinstalling it - it may not be all the way in. If this does not help, a polarizer or filter may be bad. It's going to be hard to get replacement parts for this old projector that is at least 11 years old.
The only other thing I can think of is to try a factory reset - look through the menus until you find this.

reset to default and try again

Are you simply feeding the projector with a video signal outside the range the projector can accept? For instance, trying to feed HD video to a projector that is not capable of high resolution can produce an out of sync image - or nothing at all.

I assume you've tried using a different video cable to connect to the projector, just in case there is a broken core in the original one that carried the sync signal?

For a worst case scenario you may have to reinstall your Operating System. But try installing Auslogics BoostSpeed and running the diagnostics. Have you installed anything prior to the problem? Sometimes new software can corrupt old system files when you install. Try uninstalling the last software you installed. Hope this helps.

If problems arise while installing or uninstalling the software, download this removal utility to uninstall the HP All-in-One software before attempting software reinstallation. IMPORTANT NOTE:
This utility removes only version 4.2.0 of the driver.
Running the removal utility may remove the drivers and software for other HP printing, scanning or digital imaging products.
This utility works in stages. The first time the utility is used, it runs at level one. The next time the utility is used, it runs at level two. There are four levels. The effects of each level are shown in the table below.
Uninstaller level Description 1 Removes the Windows device discovery (Plug and Play) entries from the Windows registry. After running level 1, connect the USB cable to the all-in-one and let Windows try to discover the device again. 2 Performs Level 1 and removes the software drivers. After running level 2, run the all-in-one installer and use the "Add a new device" option to attempt to reinstall the all-in-one again. 3 Performs Levels 1 and 2 and removes the HP Photo and Imaging software. You must completely reinstall the all-in-one software after running level 3. 4 Performs Levels 1, 2 and 3, and removes HP Software Update. You must completely reinstall the all-in-one software after running level 4. Installation instructions:

1. Click the Download now button.
2. When prompted, choose Open or Run to start the download and run the installer.
3. Follow the onscreen instructions.

Details and specifications File name: gr_uninst.exe [1/1, 866.52k] Released: 2005-01-11 Version: 4.2.1 Compatibility: Microsoft Windows XP, Microsoft Windows 2000

reinstall the lamp door. ,that cover that you removed to acces the lamp.

go to the menu , check information or lamp timer . u will get the lamp hours.
u can also physically check the lamp for melted filament, white deposit on reflector of broken filament to confirm

it works if you do it in bootcamp. but have to select external monitor instead of digital display under the intel graphics control panel. does anyone know of such an option under OSX?

I had the same problem, reset the lamp hours by pressing 'menu, 619 , menu" and browsing until I found the setting, but then I also saw an option to test the lamp.  The TV shut off when I tried this and 2 red lights indicating a new lamp is needed came on instead.  It's probably the ballast and not the lamp, but I don't know that I want to put any more money into this set.  I have only had it 2 years.