Im using a HDD been all fine beofre, but trying a different theme from the menue, when rebooting it hangs on the loading screen (almost complete) I have tried a few times & left for well over an hour.
booting without the HDD starts up.

recalbox seems a bit buggy, as i previously had it so it would do a full check of all roms etc everytime (taking 10 mins every boot).
I wiped the HDD & started from scratch but just as I was getting it just how I wanted this happens.
is there a way to edit the theme change perhaps to get it to boot again?

did you use the installer from this website? Id recommend that & get the one for the right version of Pi you have.
Try a different micro SD card to make sure its not that too.
try a different install of OS from the installer, to test the PI is working too.