So your Cool-er froze on the grey gradient screen that you normally see when you turn it on?
Yeah it happens, and is a known issue. If you have the orignal firmware that Cool-er was released (pre v1.12.6) with this is a common problem. Good news? They seem to have managed to fix it in later versions of Cool-er firmware.
If you have tried powercycling and resetting it using the little reset button the back it might be time to go for the firmware update.
See:
http://www.coolreaders.com/fw.asp
For the latest firmware. You will need a SD card in order to do the firmware flash, apparently you can't do it through the USB cable at this time.
If your Cool-er still does not boot up properly at this time you may need to contact Interead about the problem.
Amy