On a physical server I would say the disk is failing. On a VPS there could be several underlying causes, but what it really means is that the kernel is receiving I/O errors in the disk subsystem.
You should contact your provider to get the root cause fixed.
You should also copy all essential data off of the VPS because you may be on the verge of losing it (but it's hard to say without knowing more details about the VPS underlying system).
Once you've copied your data off a reboot should bring the filesystem back to read/write. Be warned though, it might not be able to survive a reboot. It may be down hard if you try.
Again, contact your provider.