a/c has been working fine until yesterday afternoon, turned it on and nothing happened, no fan and no outside compressor. I put in a new tstat and still nothing. I left it in the auto a/c position and went to bed. Sometime in the middle of the night the unit came on and was working fine when we awoke the next morning. Turned it off and went to work. However, when we came home this evening, turned it on and again nothing happened. Any suggestions.

Thanks, mark

If this is a central air unit, a wild guess would be the thermostat, since it appears to work, just not when you want it to.