What does actually happen if you try a hard-reset: by which I mean, holding the power button down for *a while* (~20s, possibly more. More certainly won't hurt anything besides your finger ^^)?
If you had the FL on, a good hint that the battery cutoff took is that the frontlight will turn off. You can then release the button, wait a few seconds, and just press the power button normally (which is slightly longer than the usual short suspend/resume press ;p) to power it back on.
(Since you have nightmode enabled, a good bet on what happened is an EPDC hang, Mk. 7 kernels have known issues in corner-cases not really exercised in Nickel, and nightmode is one such aggravating factor. In these instances, a hard-reboot should, well, reboot it just fine, though
).