Never move the switch from on to off or from off to on without stopping it on test and waiting for the test message to show up on the LCD This may take several minutes if you have the board really screwed up.

Put new batteries in the unit and a new SD card. Switch to test and wait until it says test, then move the switch to on.

Same method when you turn it off. NEVER TAKE THE CARD OUT WITH THE UNIT POWERED UP. Your SD card stands a 50/50 chance of being locked up. Same way with your computer. Never pull the card out of the reader without shuting it down on your computer and waiting for the "safe" message.

I have four 0.3 MP units and they work great if you use them exactly as they are supposed to be used. I made the same mistakes and have locked them up and ruined SD cards before.