So I beat the game the other day, but after I beat Sequence 14, it didn't give me an achievement, and after I escaped the hideout in the credits it didn't give me an achievement, and right after that, i checked my gamer profile and it said I had NO achievements for ANY game.  I signed out and signed back in and my achievements came back, but I still didn't get the Sequence 14 or Escape the Hideout achievements.  Since there's no replay button for missions, I don't really know what to do.  I guess I just get shafted for 200 points.  I mean I know they're just achievements, it just ticks me off a bit. 
 
Anyone else have this problem and know a fix?

It's been a while since I dived in to Assassin's Creed, but can't you reply memories sequences from the menu? Just browse the DNA chain to the proper sequence and try opening it.

Maybe there was in AC1 (I never really played it) but no, there isn't a replay button in AC2.

Yes, you can replay missions. 
 
Go into DNA, look for the specific strand of DNA that you are wanting, and then hit A.  Once you do so, it'll give you the option to replay the mission. 
 
Unless they changed it since I got my S rank on it...  =  /

You can only replay certain missions, e.g. assassins tomb ones. Also, I had a similar problem with the game not giving me achievements. There is one achievement you can get for buying a painting in Florence and Venice, I did this but the game did not give me the achievement.