Before you continue below, make sure you’ve first tried a hard reset to revive your router:

1.  Disconnect the router from UTP cables (not the power cable).
2. Push reset button for 30 secs.
3. Without releasing reset button, disconnect power cord.
4. Hold the reset button for another 30 secs.
5. Replug the power cord.
6. Still hold the reset button for another 30 secs.
7. Release the reset button and give the router about 10 secs to resettle.
8. Disconnect power cord for another 10 secs and then reconnect.
9. All should be in default settings now.

This procedure is usually called 30/30/30 reset.
If the power light blinks in a neverending way, the 30/30/30 reset has no effect.

Seize The Moment With TFTP :

You can use the TFTP  (Trivial File Transfer Protocol (define) command-line utility to
upload fresh firmware to the router in its brief moment of wakefulness.
Windows, OS X and Linux all include TFTP clients.

First, you need a .bin file containing the known good firmware. You can visit the Linksys
site as described above and download the .zip version of the firmware,
which includes the .bin file.

You can even use a .bin file for an open source firmware, so long as you choose
one that’s stable and tested and is the correct version for your router
(or else you start this whole process all over again).

Windows users need some dexterity here. First, remove power from your router.

Open two command prompt windows. In one, you will set up your TFTP command.

Type (but do not yet press enter) :

tftp –i 192.168.1.1 PUT firmwarefile.bin

In the second command prompt, enter:

ping –t 192.168.1.1

Run the ping command, which will begin probing and failing to reach the router.
Now change window focus to your TFTP command.

Apply power to the router. Watch the ping window for a response,
Then hit enter in the TFTP window !

Miss by a beat and you’re too late – the pings will fail and your moment has passed.
Cut power to the router and repeat the process.
You need to start the TFTP the moment you see a successful ping.

For OS X and Linux users, the principle is the same, but the process is easier.
First, remove power from your router. Open a terminal window and enter

the commands:

tftp 192.168.1.1
binary
rexmt 1
timeout 60
trace
tftp> put firmwarefile.bin

Now apply power to your router. The tftp client will continuously retry uploading the firmware
until the router responds. Hopefully, the router will briefly awaken, allowing the firmware upgrade
to be sent. About two minutes later, the router will reset and become operational with the new firmware.

Notes :

Enabling boot_wait

The router does not boot directly into the firmware, instead it boots into a program
known as a bootloader which is responsible for initializing the hardware and loading
the firmware. If the boot_wait variable is set, the bootup process is delayed by few
seconds allowing a new firmware to be installed through the bootloader using tftp.

Recommend that you use boot_wait for your first install. This will confirm boot_wait
is correctly enabled and provide a firmware recovery experience without the stress of
a broken router.