I'm having trouble uploading code to my arduino

It was working before, but now I keep getting an error that says " avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_getsync() attempt 10 of 10: not in sync: resp=0x31".

What model Arduino?

If it is an Arduino UNO, what are the RX and TX LEDs doing when you ty to upload?