The tortoise may move in circles or hold its head at an unusual angle. One or more of these symptoms may be present in any combination. If you hibernated your tortoise in a garden shed for example during the extreme cold of last winter and it now displays symptoms like these, there is a very high probability that it has indeed sustained frost damage. Note that this sort of eye damage requires expert diagnosis - without a proper examination using special equipment the eyes may appear to be perfectly normal. The prognosis is very variable.
Some cases recover completely reasonably quickly, other can take years.
Some never recover. It very much depends upon the severity and extent of the damage sustained. Emergency first aid Dehydration is the most immediate danger. Fortunately tortoises do not find this as distressing as mammals, and from the owner's point of view the procedure is both safe and relatively simple. It is frequently highly successful with sight-damaged animals.