Check Engine Light Codes Reinstallation?

I have a 1999 Honda Civic Si and Im not entirely sure whats wrong with it. The check engine light has been one for awhile but we just now took it in to see what exactly was wrong with the car.

When the mechanic checked it however, the code did not appear.

He said that it looked like the codes had been erased.

Im not sure how that happened or could have happened, but the battery did lose connect once while the light was on b/c of a missing screw.

Could the temporary loss of connection with the battery have erased the codes?

The light did go out for awhile after that but eventually came back on.

The mechanic who checked the car also turned off the light to see if it would come back on, which it did so we'll be taking it back for him to look at soon.

Im just wondering if anybody can shed some light on what possibly occured?

And sorry for the long post.