I'm not a commish, but it's in their Commissioner's toolkit. They can lock any one of several functions: daily roster adjustment; trades; drops/adds/free agent bids; posting a message; changing the team name. I may have missed one (I acted as commish when the real commissioner was incapacitated-never had any use for those tools but saw they are available).
I imagine these exist for rare circumstances such as abusive communications or total abandonment of a team. I would contact your commish for an explanation. If that is not a viable option for some reason, contact yahoo fantasy league office customer support to see what your options are (I don't know if you can file a protest like you can on an unfair trade or trade veto).