You have copied the program startbl.exe from an upgrade file to your device and your started it on your device
You were upgrading your device (ROM upgrade for example) and something went wrong
You were holding the power on/off button and the record button and you pressed the reset button (tiny hole left from the battery lock)
Case 1: I was not performing a ROM upgrade
If you were not trying to upgrade to a newer ROM version then you can try to reset the device (press reset) or hold power button and press reset or hold power button and camera button and reset.
Case 2: I was performing a ROM upgrade
Note: Be patient, it sometimes takes a while before the actual upgrade starts.
If nothing has changed or happened in about 10 minutes, try this:
Make sure your device or cradle is connected directly to your computer (so no hubs!)
Disconnect the device from the PC, or remove from cradle
Perform a "hard reset" on the device (screen will flicker but nothing will change)
Kill the BaUpgradeUt.exe program on your PC by using Window's Task Manager (CRTL+ALT+DEL)
Kill the wcesscom.exe program on your PC by using Window's Task Manager (this is the ActiveSync application)
Place the device back in it's cradle
Restart the upgrade program from your PC (double-click BaUpgradeUt.exe)
If your device seems to hang when in bootloader (nothing happens for hours while the upgrade program is checking your device) remove your SD-Card and try again. This problem can occur with large (> 1GB?) cards