If your HP Scanjet 5550c is not detected by your scanning software, here are a few things that you can try to resolve it.
Check the LEDs
First, go ahead and check the LED near the power button to see if it is illuminated to show that it is receiving power or if there is an issue that is letting the LED flash rapidly.
Check the Cables
Make sure that the USB cable between the system and the scanner is connected at both ends. If it is connected, please disconnect it for ten seconds at both ends, then carefully reconnect. In Microsoft Windows, you should hear a connection sound played if you have audio enabled.