ok
i had the same problem a few days back.
There are 2 aluminium plates on the monitor mainboard.
one controls the horizontal ratio and the other does for the vertical.
now that u mention that there is only one horizontal line in the centre; so ur vertical ratio isnt correct.
wat i did was just press the aluminium plate back to the board hard . yes try it ; it does the work but only temporarily.
for permanent solution u need to do soldering.
call up some computer mechanic and on the max. he/she shud charge Rs 300.