I can't get the minecraft admin and op powers to work,

I am unable to get the op and admin powers to work for minecraft. I was somehow able to last night, but I can't now for some reason. I have tried typing op into the server gui, along with restarting the server and other things. I got them working last night, disconnected, and when I opened the game again this morning, they weren't working. It would be really appreciated if I could get some help solving this problem.