Transmission won't go into gear

I had to put my car up on ramps
to change  a alternator belt. But
when I went to get back down it
would go in reverse. I had to push
it off.  Then it wouldn't go in drive.  
But it would in reverse.
I shut it off. Tried again but then
wouldn't go in either.
And now it only works in reverse
sometimes.
I've changed the transmission
fluid. And if I push really hard on
my brake it will go into reverse but
not drive