from one campaign to another

Yes, how is that done?
I can get my factions character to lions arch in the prophecies campaign, but how do i get it to ascalon to start that campaign?

run there

And how are one supposed to do that when every road out of lions arch is closed?

For non-Prophecies characters the campaign starts in Lions Arch and goes from there.
The only benefit of running all the way back to Ascalon would be to complete your
cartography title.

For foreign characters the campaign starts in LA, just as non-Factions characters start in Kaineng City rather than Shing Jea. You can run back to Ascalon if you want to do the newbie missions, but there will be no reward for them, just as non-Factions characters don't get a reward for Minister Cho's Estate and Zen Daijun.

To start the campaign for a foreign character, just talk to everybody in LA who offers a quest. "Report to the White Mantle" from Armen the Guide is the primary quest that will lead you to the next mission area in D'alessio Seaboard.

I'm not sure what you mean about every road out being closed. I took a Factions character to LA just yesterday, and had no problem walking out the front gate. (It's to your right as you face Firstwatch Sergio on the stairs.) Once you go out the gate, just turn right and head east. It's quite a long trip to Ascalon City, but there are many places to stop on the way.

Edit: One thing to note, in case you didn't know. Unlike Factions, you are not required to take any quests to get into a mission outpost. You can travel and enter outposts freely, and you can do any mission you find there even if you haven't done any preceeding missions or quests. Of course, it's easiest just to stick to the story line. But, there are a number of outposts that aren't visited in the course of the story, or even as a result of side quests, and you won't reach them except by exploring around.

nice, thanks

Although, keep in mind, you might want to get to those outposts in Ascalon, Shing Jea, etc, because you will (hopefully) eventually want to go for a protector/guardian title, cartography title, vanquishing, etc, and you'll need those areas on your map. So I'd get them just to save a minor headache later.

Ummm... Protector... Guardian... Vanquisher... zquests... Nicholas

Titan Quests, Grim Cesta, Moa Egg, Legendaries .... the list goes on

Best thing to do, if you want to go back to Ascalon, would be to download a cleared map from a wiki. Then go from L.A. to Gates of Kryta - then to Beacon's Perch - then go north to Ice Tooth Cave - then east to Yak's Bend. From there it's a pretty basic path back to Ascalon.

Unless the character is Tyrian (from the Prophecies campaign), the quests in Ascalon will not be available, and as such the OP won't be able to get the Cesta.