Check the ACT LED to determine status of the PI. RED light on, green light only activates if it accesses the sdcard. Both lights solid on mean no detection of SDCARD or invalid image.
Determine boot status of PI:
Method 1:
Unplug both the USB and USB-C cables. Plug back in USB-C and USB cables after 10 seconds. The green LED is essential in determining if the Pi can actually read files from the card. If the green LED doesn't blink quickly in an irregular pattern for at least ten to twenty seconds, this indicates that the Pi cannot read from the SD card, and that booting is not taking place.
If both the RED and GREEN LEDS are on and never flash, it did not detect the SDCARD. It doesn't matter if the ACT LED initially starts on or off. Only the blinking of the ACT LED that is significant. A dim ACT LED also has no significance beyond the fact that you are using a revision B board. The LED is driven differently and won't be dim in Revisions B+ and onwards
The default behavior of the LEDS when booting would be green and red lights on, then green begins to blink randomly, if any errors are encountered, the green will blink a code and repeat.
Method 2:
Attach a monitor to any of the two micro-HDMI ports. Here you can view any error that the PI encounters
Attach a USB keyboard and use shift-page up or shift-page down to scroll through the log screens