First, the unit might be booting still. A unit without an IP address can take up to a minute and a half or so to boot, about a minute to timeout waiting for a DHCP/BOOTP server to give it an IP, and then another 15-20 seconds while it decompresses the OS into RAM and runs. During this time, the LED will be flashing yellow or orange.

Second, the unit might be in the firmware monitor. Normally the unit will only go the firmware monitor after you press the reset button. But it can be configured to stop in the firmware monitor every time it boots. Try cycling the power on the unit and see if it boots.

 Third, there might be a hardware problem. If the unit detects a fatal error during self test, it will blink an error code on the LED. This will be a two digit error code. For instance, a 92 error (memory error) will be displayed as 9 blinks, slight pause, 2 blinks, longer pause, repeat.