There are two possibilities: A. you've massively underdeveloped and B. you've underdeveloped a little bit...
In the first case, there's not much you can do (there are some solutions, but complicated and difficult to realise).
In the second case, you will not have lost your shadow detail (shadows do not need much development to show on the negative) so you just have to correct the negative contrast for it to be printable on an acceptable paper grade. Try selenium toning. It's relatively safe and sometimes effective. Other options are chromium intensifier, that is a quite dangerous poison and can also ruin your negative. Mercury intensifier is not an option, in my opinion. Too dangerous for your health... so, if someone suggests that you use it, just forget it.