Umm, yes, quest XP stays the same, but that doesn't seem to be the issue here

I've never heard of quests just disappearing from the journal for no reason - do you remember which ones they were? What zone are you in? You should be able to re-acquire any quest that you have not completed just by going back to the place where you got them from before. The only time where this would not be possible, and may explain your situation, is that some "pointer" quests are no longer available after completing the quest they point to. In Stonefalls, for example, it is no longer possible to do "A Higher Priority" (from Fort Virak) if you have already done "The Coral Heart" (at Vivec's Antlers).

The other possibility is that they are still there, but in a different section. For example, if you go from Stormhaven to Rivenspire before completing all your Stormhaven quests, if you open the journal in Rivenspire, the Stormhaven section may have closed. Just click on it to open it up again. Similarly, if your active quest is a chapter of the Main Quest, your current Zone section may also be closed.