Some weeks ago I had the problem that I couldn't flash a firmware. Neither the web interface nor the Asus restore tool worked. I found out that following rescue procedure always works (pc connected to asus by cable of course):
I also posted this in a German forum and all users with the no access problem could recover their unit.
1. pull power cable and wait 20 seconds
2. keep button pressed and connect power cable: power led should flash
3. ping 192.168.1.220 should reply else redo steps 1 and 2
4. run Asus Firmware restoration tool, select firmware file (e.g. WLHDD_188.8.131.52_en.trx) and press upload. After some seconds press the Abort button or wait until it says "Upload fails! Please check the connection"
5. from a dos box or linux console run: tftp -i 192.168.1.220 put WLHDD_1.1.2.x_en.trx
6. tftp returns something like: "Transfer successful: 3731456 bytes in 2 seconds, 1865728 bytes/s" if it doesn't redo steps 1-5 (sometimes I needed three or four times before it worked)
7. after ca. 80 seconds your Asus should reboot