With the Vita off take out both the SD2Vita and the Vita's memory card and then turn it on, let it boot then turn it off.
Next insert just the memory card and turn the Vita on while holding the PS Button, R trigger, and power at the same time to enter safe mode.
From the safe mode menu try a rebuild database, and maybe even a format if your card doesn't contain anything important. Power off and try booting normally.