I think with many Linksys routers you can use a utility called "TFTP" to force a firmware flash on routers that have died due to dodgy flashes.
I'm not sure but I think that utility may work with other makes of router (obviously it depends on how badly messed up the router is), as I think it just basically pushes the firmware to a specific IP address on your network - and it seems that some routers (Linksys at least) have a "boot block" type bios that isn't affected by normal flashes and allows for recovery via this method (the same as a lot of motherboards now can still accept a firmware update from floppy even if a previous bios update has stopped it booting)
The term i've seen more than a few times to describe routers in this state is "bricked" (because it's little better than a brick at that point?), so maybe a google will help?