Likely, you corrupted your SD card, and now your Anbernic is stuck in FEL mode. Just press the reset button a few times with everything turned off, and then it should be able to boot normally.
These handhelds are pretty hard to kill because the OS is on the external card. Also, MuOS is pretty sensitive to being hard-reset like that, so if it locks up again, try L2 + R2 + START; Mu has a built-in safe restart.
^ This is from my issue; but corrupted SDs, incompatible os and booting without an SD card all cause this behaviour, and it’s fixed the same way.