Why can't my dog wag or lift his tail?

My dog couldn't lift or wag his tail from one day to another....... he has a flaccid tail, but there is no pain at all; must likely is numb... Can someone tell me if is something bad? I read about "cold tail" but it said that it's painful which is not my dog situation.....in another hand they also told me that it would heal on his own... is it true? Thanks... and please answer ASAP