Hi Rej,
Some of the info I’ve been able to find out online about this suggests that for older Pi’s (before the 4) a solid red LED with a single green flash is related to an [link|https://support.pishop.ca/article/33-raspberry-pi-act-led-error-patterns|incorrect SD card image or the wrong size card] (4GB or bigger seems to be required). For the Pi 4 though, this doesn’t seem to be a known error code. I would recommend verifying the version of the Pi that you have is a Pi 4, and then trying to [link|https://projects.raspberrypi.org/en/projects/raspberry-pi-setting-up/2|create a fresh SD card following this guide.]
If this doesn’t work, it’s possible that there’s a hardware problem with the Pi. Take a look inside the MicroSD card slot to see if anything is stuck in there. Otherwise, an ESD event could have killed it sometime in storage.