Upon finishing "Dawnguard" I am trying to get out of Dimhollow with Serana. The gate that I walkked through to get down to the crypt floor is now closed and I find no way to open it - or any way around it.
Need HELP!

red_strider

Do you mean that door sized gate at the top of the stairs of the room Serana is found for the first time? If so, as you walk into the room to your left you should see a lever, this activates the gate.

You can't go back out the way you came. There's a whole other way out. Look around where you discovered Serena.

Just start moving in the opposite direction. Pretty much all Dungeons save one or two in Skyrim have a backdoor to save you the pain of backtracking. Hell 2-3 that I can remember even force you to take the backdoor way out by making part of the dungeon delving involve going down a shaft with no way back up.