2005 Hyundai Sonata (manual transmission): Car won't start after check engine light came on and car stalled.

When I started the car the check engine light came on. Since there had been a lot of rain recently, I thought that perhaps it was an electrical malfunction since the performance of the car did not seem to be affected. I was only a few miles from home and going at a constant speed on the highway. When I slowed down to get off at my exit I noticed that the car wasn't getting up to gear with the same power as it had been. By the time I had to slow down for five or six stop signs the car was having a lot of trouble getting up to speed, and at the stop sign before my drive way, it stalled and won't start (although it always sounds like it's just about to turn over). The mechanic is suggesting an engine flush and tune up, but both seem a little excessive, especially the engine flush. Any ideas?