starter motor or something else?

My car didn’t start properly. You could hear it was trying really hard to start. Over time it worsened and the engine light was on. So I went to the mechanic, they ran some tests and found that some sensor was malfunctioning? They also suspected something was wrong with the starter motor.
However, they reset the system and my car was perfectly fine after that ( did not replace anything)
The mechanic told to come back if it happens again…
And it has started happening again….
I’m just looking for some ideas of what it could be?
If it was the starter motor malfunction how could it start once the electrical system was reset?