Probably you restarted your HDD too early (before the last write access was finished) and now it is validating. Wait until it stops validating (HDD LED goes dark), then it should become writable again automatically.
If that does not help, open a shell window and enter Info. What is the status of the partition ?
At last get the Check4GB archive from Aminet, run the CheckHD program and post the output here.
However, you could try to enter the command Lock DH0: OFF in a shell window (DH0 should be replaced by the name of you partition). But this does not solve the problem.
Bye,
Thomas