Assuming it's a Yamaha, (if not, other brands use a similar lever set up). It's a black plastic lever with a bail type return spring. It has a variable resister inside. They often freeze up.
Here's a diagram, ("trim sender", item #3) -
http://www.boats.net/parts/search/Yamah ... parts.html I used a 2002 200 hpdi as an example and I think most Yamahas are the same.
It's mounted inside the motor bracket, behind the tilt tube. You'll need to tilt the engine up all the way to get at it. Spray crc, wd-40, etc, into the pivot area, (use a nozzle stem). Try to work the lever back and forth to free it up and repeat spraying with lubricant. You might need a long screw driver to reach it.
Also, there's a small black plastic sleeve, ("cam, trim sender", item 1) with a tab, mounted on the tilt tube. The lever arm rests on the tab while trimming. It's adjustable, so you can accurately set the trim indicator range. Make sure that isn't lose or broken.
I'd say 99% of the time it's a frozen lever, and from what you desribe, (1 bar) it sounds like that's the case... it's stuck in the down position.