Tranny fluid and pan filter are the things to look at here.
This happened similarly to me once, I went out, fired up the car, put it in drive, but nothing seemed to happen!
I tried the gas, and the motor coughed a bit but eventually reved up - but I didn't seem to be getting any juice, not even enough to reach 2mph.
So I tried putting the transmission in neutral and then back in drive, and it was OK.
It was a friend that suggested I try to get a pan service or fluid top up. I mentioned this in the shop, and they verified it. I am now running my motor right as rain again!