You must have had a power surge that fried your internal memory.
The reason I say this is that you said that when you came back to the MPC it was on the memory check screen. The MPC only does a memory check when it powers on, so I'm guessing your power went out (at least for a sec.). Then when the power came on, it surged, which is pretty common, and fried your memory.
Don't tell AKAI that you left it on though, or that you might have had a power surge, because I'm sure they won't cover that under warranty. Just play dumb, and they might replace it.
This brings up an interesting point that I had never thought about before. The power switch on the MPC is mechanical, as in, it locks into the on position when you press it (as opposed to a switch that "activates" the power, but does not stay in an ON position). So, if you leave your MPC on, and the power goes out, of course the MPC powers down. But since the switch is clicked into the ON position, when the power comes back on, the MPC is still in ON mode, so any resulting electrical surge can potentially damage the MPC.
That right there is a good reason not to leave your MPCs on for long periods of time: POWER SURGES. I never thought of that.
