If you don't have a reset switch then try holding the power button on for 10 seconds...
If still no luck thry connecting the gps to your PC using a usb lead and see if it will go into PC mode.
If it does leave connected for a couple of hours before removing and try powering up again.
If it does go into PC mode but still won't power on then we need to update the unit software:
It is possible your firmware has got corrupted and this will correct it...
Go to this link and download the webupdater software, run the software and connect your GPS to the computer via a USB lead when prompted.
http://www8.garmin.com/products/webupdater/howtoinstall.jsp
If the unit still won't start then your unit has an internal fault and will need to be sent to Garmin for repair..
Engineer
NZ Certificate in Engineering Electronics & Computing 25 years exp