After cet took the lich i went op the stairs to the next room, i got the treasure out of the chest and tried to open the large door, now it is jammed, is there anyway to open it?

the door is jammed when you failed to often to open it. You need round about piclock skill of at least 7 (better 8) and a level 5 lockpicking set.

but there only some dull treasure behind it, nothing important.
you could also get the key from alissa(don`t know the name) the vampiress who gave you the order to find a scroll (reward is a ring or the key)

but as far as i found out, you can`t open jammed doors when you failed too often