In general, if a Philips Digital Pocket Memo is locked up, removing the batteries for thirty minutes may reset it (soft reset).
The Philips support group in Vienna suggests removing the batteries overnight in stubborn cases. At that point, you should be able to update the firmware (if needed) or perform a reset (through the Philips SpeechExec Pro software).
If the problem continues, the units should be replaced.
Verification Source: Steve Kemble, Philips Support
Test Date: 9.23.11