On my server, on my first day playing, I was accused of griefing. I had been exploring the world, and saw an unprotected (unclaimed) house all messed up, so I filled in the holes and spent the night there hiding from zombies. When the house owner complained of griefing the next day, the wonderful Addstar mods were able to see exactly what had been done and that I hadn't caused the holes, I had just filled them in, and they all apologised and gave me presents. However, because I had filled in the holes, they couldn't restore his house to how it used to be before someone else had griefed it - they could only go back to undo the last actions, and the last actions were mine. So, I learned not to 'help' griefed houses. Don't know if any of this story helps you at all, but talk to your mods via their website, and if you are quick enough about it they might be able to help you back on and restore the person's griefed property.
It is an AWFUL feeling, I feel so sorry for you.
Addstar has grief protection, which you should always use if you can. I don't know if all servers have it, or if yours does / did.
If you have no luck, just try a better server, there are heaps of them about.
Sometimes a fresh start is fun in Minecraft!
Cheers
SyrusMcClean