Will my problems never cease?

Anyway... my check engine light came on again after cleaning the mass air sensor.

I got an OBD II computer (neighbor had one, go figure) and checked the code. It was a P0440... which is an EVAP system problem.

My question is ... how/what could have happened and how do I fix it?

I cleared the code, and filled up with gas in case I had a bad tank. So far, no check engine light, but I'm expecting it to come back on soon...

Any ideas would be appreciated.