try this...

http://www.looknohands.com/chordhouse/piano/

dat be happenin to me too somtimes, but i think the best way to get over it is to just play around wit the keyboard until you fall upon somthin you like......writers block is just a mind thing to me.....just keep at it...you ever noticed when you come from writers block  your beats sound better?

i just mess with the sounds i got...  and some always happens... 

If you use a four chord progression instead of a two chord progression or one-chord...non-progression, there is less pressure to create very independently interesting melodies because the rest of the beat is so interesting.  When i've got writer's block, i sit at the piano, play chords with the left hand, and hum to myself or mess with my right hand.  Crazy, but it works.

Im feelin you mane... That chordhouse site that dude posted up to is really handy.  If thats no help then dont make a beat, Just do a clasical composition, without drums and stuff...just straight instruments.  That helps me most times.  If you go to my soundclick page ( www.soundclick.com/nyquist1 ) and look at Sweet Soliloquy, Glimpse, and Piano Piece.  I did thoes to help with melodic writers block.  Oh and If you just listen to every style of music you can get your hands on ( every type of music from around the world) I guarantee that will help.  If you use that aproach though, try to get into the music. I mean listen to the different kinds of world music for a month or something, and then copy some of thoes melodies.  Or you can just literealy take a break from beats for a little while.  and come back to it.  That helps too. 

Just give it time man. It'll come back and when it does you'll be a little better. Until just listen to alot of music and get ideas from other producers DON'T BITE THEIR SH*T!!!!!! jUST GET IDEAS!!!