Welcome aboard!!
Check the pressure on the bow while we're at full speed.
Make generous use of the tutorials. Use them for more than just what the mission tells you, just use your imagination. I still go back and visit them on occasion when I want to try a new technique or test something I read.
We need more information to help.
What version are you playing? v 1.5, otherwise known as the U-boat add-on, is the latest stock version. The earlier versions had a lot of bugs.
Have you added any mods?
Are you diving too deep? I don't remember the maximum depth for the subs in stock. On the deep dive guage on the toolbar, there is a marker for the test depth. In stock, that is normally the deepest you can dive.