Warning light

2 days ago the I struggled to start the car and once I got it going the warning light of a car with a spanner through it came on and the car was sluggish. I stopped the car and turned the engine off and back on again and the light went off. This happened to me again this morning. The manual says that if the light comes on you should try to turn the engine off and on again and it might fix the problem which it did. However, this doesn't seem like a real solution to a problem to me and the fact that it's happened twice in 3 days makes me wonder. I think the car might be a little sluggish but I could be imagining it. It is overdue a service and a lot of these forums suggest that this might be why the light is coming on. I'm looking for some genuine advice before I put my hands in my very empty pockets. Any help would be greatly appreciated.