this happened to me once, and it turned out the internal battery was as dead as a dodo. I had to plug it in for a few minutes before it came back to life.
have you tried the reset button ? caught me out big time , screen froze etc...on my TomTom go its a small hole by the mounting plug , needs something fine like a paper clip to do it...took me ages to find this out, its no where in the manuals..later ones or certain models are marked where the reset is...
Is it possible the SD card has a dodgy or corrupted program ?