You bought a computer that was designed for people who can deal with this sort of problem; I thought that was pretty obvious when I bought mine.
Have you tried leaving it on? I don't know what OS you're using, but sometimes booting takes a little longer. Else you'll need to either boot from an SD card to recover the system, or, reflash the eMMC with an adaptor in another computer.
I'm not sure if this is your particular issue, but I had at some point left a bootable SD card inside and forgot about it.
After a reboot I could not boot my device. I only found out about it because i wanted to boot from another SD card that I found out
there was already one inside it :facepalm: