I tried searching for this and got nowhere (I think the terms are too generic so I get hundreds of hits on all kinds of things)...
I just attacked a convoy in a type VIIB uboat. I was gliding in ahead of the convoy's track, looking through the attack periscope and had to pause the game for a second. When I came back, the entire bar across the bottom of the screen (the one which has, left to right, the officers, speed, direction, depth, time and game speed) was just gone. It was not there anymore. It wasn't just invisible, I tried clicking around near it and wasn't getting any changes. The ship identification book still showed up, but the command bar was gone.
I tried going external view and back to internal, log book and back to periscope, pausing, accessing the options menu, etc. I went to the map and the map controls on the left and top sides of the screen were all there and working.
I also noticed that the words "game paused" would not appear when the game was paused.
Any thoughts?
FWIW, I will check it out again later... as usual, I saved right before committing to the attack

. I saved, on the surface, and the problem occured maybe two or three minutes later as I approached underwater.
It's a shame, too...I still managed to put a fish into a V&W DD and left him limping around at 5kts with his butt low in the water. It was a promising opening!