Hi.
Is there a WiFi button at the back of your TD-W8951ND? If yes, make sure that you have switch it on.

If still the same, you can reset the modem in this way:
1) Turn off the device.
2) Use a pin to keep pressing the reset button for 15 seconds, and at the same time, power on the device.

After that, if the WiFi LED is still off, I'm afraid that the modem may have hardware issue. For warranty, you can double check with the shop where you buy the modem.

I bought a Tplink router TD-W8951ND for adsl use.. It was working fine for a couple of months. now i got a big trouble, I cant access WIFI signal or i cant configure WiFi when ever i try to reset and configure the message comes "ERROR: FAIL TO UPDATE DUE TO... IP format error." i can surf internet only through LAN not via WLAN. In interface there only shows Internet and LAN not WLAN.. Some couple of days after wifi comes and automatically stops .. Help me