99 sentra a/c problem
 

ok car died a few weeks ago it was the alternator, all fixed. now, the a/c is not working. took it back to used car dealer he added freon, said it didn't need much and it worked for one day. now, when i turn on the a/c there is a hissing sound and it blows hot air. can anyone tell me what this could be? hissing sounds like it's coming from vent/dashboard.

thanks for any help

I think the freon doesn't installed properly.