If you have a valid version of IOS in flash memory, then you probably only need to change the config register.
do the following commands
<b>DO NOT SAVE THE CONFIGURATION</b>
Then power off the router for 30 seconds, and back on again.
It should then go through the normal boot procedure, and you get the normal prompt.
If it fails to boot, then the most likely is that the IOS image is corrupt. You need to copy a new version to the flash in (boot) mode, just the same as normal, using a TFTP server.
If you need help with that, please post again and one of us will reply with the instructions for that.