I assume you've tried the obvious like removing and reseating the card, ejecting and remounting it? If so then it depends on what's wrong with the card.
The first thing to try is see whether another device can read it: a different phone or a computer (using a card reader - they are cheap). If it can then you have a way of backing up, then you can try to get it working in your phone again (reformatting if necessary).
If it can't then I'd definitely try a card reader in a computer and see whether some file recovery software can help. It will depend on what the problem is: some faults almost any file recovery software will handle, others may require a bit of searching or trial and error (e.g. a master boot record failure will stump a lot of utilities). But at least with a computer you should be able to find out what the problem is, rather than it just not mounting (which is about all a phone will tell you).
I'm also assuming that the card is not encrypted and that it is not formatted as internal storage. If it is then your only hope is that the phone will start reading it again.
Microsd cards do fail, so you should always keep backups of anything on them.
