I had the SAME thing happen to my unit until finally, I did the old standby – I tapped it gently on the table, and voilà, it started up again. Go figure. (not a techie, so I don’t know what actually happened).
This may sound ridiculous, but I actually bought TWO Sansa Clips, and I put ALL of my data on the tiny external drive because I was always afraid that this thing would die, and I would lose all my data (hundreds of songs, tons of audio books, podcasts for work, etc.). So when Unit #1 died, all I did was take out the microSD and insert it into my “backup” unit.