Help!! Adurino Uno Not syncing avrdude: stk500_recv(): programmer is not respon

I am completely new to adurinos and have extremely basic knowledge. I bought 2 unos recently and they were uploading fine until yesterday. When i tried to upload a new program to them it gave me the error message above. I have made sure it is in the correct port and is under the proper board settings. I have looked up many support site and have not been able to find a solution. When i plug in the uno to the usb, the on led light turns on and the l led turns on and stays on. When i hit upload, the rx light lightly blinks for a second and stops.

Please help.

Have you got anything connected to serial port (RX or TX) on the board?

No i have the board completely blank.
I just tried it on both my laptop and my desktop with the same result.

Does it appear as a com port when you plug it in? (As in do you get a new port in the IDE with it in, that you don't get when it isn't plugged in?)

If yes, have you tried pressing reset just as the upload is about to start?

yes it does. i have tried pressing the restart, holding it and all different configurations of it, stiill nothing

its bootloader crashed error. when you connect arduino board TX line keep flicker more than blink example or TX line stay always high.

so kindly upload bootloader using Avr pocket programmer.